Improved build.xml

[vimdoclet.git] / sample / java.util.regex.Pattern.txt

*java.util.regex.Pattern* *Pattern* A compiled representation of a regular expre

public final class Pattern
  extends    |java.lang.Object|
  implements |java.io.Serializable|

|java.util.regex.Pattern_Description|
|java.util.regex.Pattern_Fields|
|java.util.regex.Pattern_Constructors|
|java.util.regex.Pattern_Methods|

================================================================================

*java.util.regex.Pattern_Fields*
|int_java.util.regex.Pattern.CANON_EQ|
|int_java.util.regex.Pattern.CASE_INSENSITIVE|
|int_java.util.regex.Pattern.COMMENTS|
|int_java.util.regex.Pattern.DOTALL|
|int_java.util.regex.Pattern.LITERAL|
|int_java.util.regex.Pattern.MULTILINE|
|int_java.util.regex.Pattern.UNICODE_CASE|
|int_java.util.regex.Pattern.UNIX_LINES|

*java.util.regex.Pattern_Methods*
|java.util.regex.Pattern.compile(String)|Compiles the given regular expression 
|java.util.regex.Pattern.compile(String,int)|Compiles the given regular express
|java.util.regex.Pattern.flags()|Returns this pattern's match flags.
|java.util.regex.Pattern.matcher(CharSequence)|Creates a matcher that will matc
|java.util.regex.Pattern.matches(String,CharSequence)|Compiles the given regula
|java.util.regex.Pattern.pattern()|Returns the regular expression from which th
|java.util.regex.Pattern.quote(String)|Returns a literal pattern String for the
|java.util.regex.Pattern.split(CharSequence)|Splits the given input sequence ar
|java.util.regex.Pattern.split(CharSequence,int)|Splits the given input sequenc
|java.util.regex.Pattern.toString()|Returns the string representation of this p

*java.util.regex.Pattern_Description*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.CANON_EQ*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.CASE_INSENSITIVE*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.COMMENTS*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.DOTALL*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.LITERAL*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.MULTILINE*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.UNICODE_CASE*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 


*int_java.util.regex.Pattern.UNIX_LINES*

A compiled representation of a regular expression. 

A regular expression, specified as a string, must first be compiled into an 
instance of this class. The resulting pattern can then be used to create a 
(|java.util.regex.Matcher|) object that can match arbitrary </code>character 
sequences<code>(|java.lang.CharSequence|) against the regular expression. All 
of the state involved in performing a match resides in the matcher, so many 
matchers can share the same pattern. 

A typical invocation sequence is thus 



Pattern p = Pattern. compile(|java.util.regex.Pattern|) ("a*b"); Matcher m = p. 
matcher(|java.util.regex.Pattern|) ("aaaaab"); boolean b = m. 
matches(|java.util.regex.Matcher|) (); 

A matches(|java.util.regex.Pattern|) method is defined by this class as a 
convenience for when a regular expression is used just once. This method 
compiles an expression and matches an input sequence against it in a single 
invocation. The statement 



boolean b = Pattern.matches("a*b", "aaaaab"); 

is equivalent to the three statements above, though for repeated matches it is 
less efficient since it does not allow the compiled pattern to be reused. 

Instances of this class are immutable and are safe for use by multiple 
concurrent threads. Instances of the (|java.util.regex.Matcher|) class are not 
safe for such use. 

Summary of regular-expression constructs 



Construct Matches 

Characters 

x The character x \\ The backslash character \0n The character with octal value 
0n (0<=n<=7) \0nn The character with octal value 0nn (0<=n<=7) \0mnn The 
character with octal value 0mnn (0<=m<=3, 0<=n<=7) \xhh The character with 
hexadecimalvalue0xhh uhhhh The character with hexadecimalvalue0xhhhh \t The tab 
character ('u0009') \n The newline (line feed) character ('u000A') \r The 
carriage-return character ('u000D') \f The form-feed character ('u000C') \a The 
alert (bell) character ('u0007') \e The escape character ('u001B') \cx The 
control character corresponding to x 

Character classes 

[abc] a, b, or c (simple class) [^abc] Any character except a, b, or c 
(negation) [a-zA-Z] a through z or A through Z, inclusive (range) [a-d[m-p]] a 
through d, or m through p: [a-dm-p] (union) [a-z d, e, or f (intersection) [a-z 
a through z, except for b and c: [ad-z] (subtraction) [a-z a through z, and not 
m through p: [a-lq-z](subtraction) 

Predefined character classes 

. Any character (may or may not match line terminators) \d A digit: [0-9] \D A 
non-digit: [^0-9] \s A whitespace character: [ \t\n\x0B\f\r] \S A 
non-whitespace character: [^\s] \w A word character: [a-zA-Z_0-9] \W A non-word 
character: [^\w] 

POSIX character classes (US-ASCII only) 

\p{Lower} A lower-case alphabetic character: [a-z] \p{Upper} An upper-case 
alphabetic character:[A-Z] \p{ASCII} All ASCII:[\x00-\x7F] \p{Alpha} An 
alphabetic character:[\p{Lower}\p{Upper}] \p{Digit} A decimal digit: [0-9] 
\p{Alnum} An alphanumeric character:[\p{Alpha}\p{Digit}] \p{Punct} Punctuation: 
One of !"#$%?@[\]^_`{|}~ [\!"#\$%\\?@\[\\\]\^_`\{\|\}~] 
[\X21-\X2F\X31-\X40\X5B-\X60\X7B-\X7E] --> \p{Graph} A visible character: 
[\p{Alnum}\p{Punct}] \p{Print} A printable character: [\p{Graph}\x20] \p{Blank} 
A space or a tab: [ \t] \p{Cntrl} A control character: [\x00-\x1F\x7F] 
\p{XDigit} A hexadecimal digit: [0-9a-fA-F] \p{Space} A whitespace character: [ 
\t\n\x0B\f\r] 

java.lang.Character classes (simple java character type) 

\p{javaLowerCase} Equivalent to java.lang.Character.isLowerCase() 
\p{javaUpperCase} Equivalent to java.lang.Character.isUpperCase() 
\p{javaWhitespace} Equivalent to java.lang.Character.isWhitespace() 
\p{javaMirrored} Equivalent to java.lang.Character.isMirrored() 

Classes for Unicode blocks and categories 

\p{InGreek} A character in the Greekblock (simple block) \p{Lu} An uppercase 
letter (simple category) \p{Sc} A currency symbol \P{InGreek} Any character 
except one in the Greek block (negation) [\p{L} Any letter except an uppercase 
letter (subtraction) 

Boundary matchers 

^ The beginning of a line $ The end of a line \b A word boundary \B A non-word 
boundary \A The beginning of the input \G The end of the previous match \Z The 
end of the input but for the final terminator, ifany \z The end of the input 

Greedy quantifiers 

X? X, once or not at all X* X, zero or more times X+ X, one or more times X{n} 
X, exactly n times X{n,} X, at least n times X{n,m} X, at least n but not more 
than m times 

Reluctant quantifiers 

X?? X, once or not at all X*? X, zero or more times X+? X, one or more times 
X{n}? X, exactly n times X{n,}? X, at least n times X{n,m}? X, at least n but 
not more than m times 

Possessive quantifiers 

X?+ X, once or not at all X*+ X, zero or more times X++ X, one or more times 
X{n}+ X, exactly n times X{n,}+ X, at least n times X{n,m}+ X, at least n but 
not more than m times 

Logical operators 

XY X followed by Y X|Y Either X or Y (X) X, as a capturing group 

Back references 

\n Whatever the nth capturing group matched 

Quotation 

\ Nothing, but quotes the following character \Q Nothing, but quotes all 
characters until \E \E Nothing, but ends quoting started by \Q ?[\]^{|} --> 

Special constructs (non-capturing) 

(?:X) X, as a non-capturing group (?idmsux-idmsux) Nothing, but turns match 
flags on - off (?idmsux-idmsux:X) X, as a non-capturing group with the given 
flags on - off (?=X) X, via zero-width positive lookahead (?!X) X, via 
zero-width negative lookahead (?<=X) X, via zero-width positive lookbehind 
(?<!X) X, via zero-width negative lookbehind (?>X) X, as an independent, 
non-capturing group 





Backslashes, escapes, and quoting 

The backslash character ('\') serves to introduce escaped constructs, as 
defined in the table above, as well as to quote characters that otherwise would 
be interpreted as unescaped constructs. Thus the expression \\ matches a single 
backslash and \{ matches a left brace. 

It is an error to use a backslash prior to any alphabetic character that does 
not denote an escaped construct; these are reserved for future extensions to 
the regular-expression language. A backslash may be used prior to a 
non-alphabetic character regardless of whether that character is part of an 
unescaped construct. 

Backslashes within string literals in Java source code are interpreted as 
required by the Java Language Specification as either Unicode escapes or other 
character escapes. It is therefore necessary to double backslashes in string 
literals that represent regular expressions to protect them from interpretation 
by the Java bytecode compiler. The string literal "b", for example, matches a 
single backspace character when interpreted as a regular expression, while "b" 
matches a word boundary. The string literal "(hello)" is illegal and leads to a 
compile-time error; in order to match the string (hello) the string literal 
"(hello)" must be used. 

Character Classes 

Character classes may appear within other character classes, and may be 
composed by the union operator (implicit) and the intersection operator ( and 
and ). The union operator denotes a class that contains every character that is 
in at least one of its operand classes. The intersection operator denotes a 
class that contains every character that is in both of its operand classes. 

The precedence of character-class operators is as follows, from highest to 
lowest: 

1 Literal escape \x 2 Grouping [...] 3 Range a-z 4 Union [a-e][i-u] 5 
Intersection [a-z 

Note that a different set of metacharacters are in effect inside a character 
class than outside a character class. For instance, the regular expression . 
loses its special meaning inside a character class, while the expression - 
becomes a range forming metacharacter. 

Line terminators 

A line terminator is a one- or two-character sequence that marks the end of a 
line of the input character sequence. The following are recognized as line 
terminators: 



A newline (line feed) character('\n'), 

A carriage-return character followed immediately by a newline 
character("\r\n"), 

A standalone carriage-return character('\r'), 

A next-line character('u0085'), 

A line-separator character('u2028'), or 

A paragraph-separator character('u2029). 

If (|java.util.regex.Pattern|) mode is activated, then the only line 
terminators recognized are newline characters. 

The regular expression . matches any character except a line terminator unless 
the (|java.util.regex.Pattern|) flag is specified. 

By default, the regular expressions ^ and $ ignore line terminators and only 
match at the beginning and the end, respectively, of the entire input sequence. 
If (|java.util.regex.Pattern|) mode is activated then ^ matches at the 
beginning of input and after any line terminator except at the end of input. 
When in (|java.util.regex.Pattern|) mode $ matches just before a line 
terminator or the end of the input sequence. 

Groups and capturing 

Capturing groups are numbered by counting their opening parentheses from left 
to right. In the expression ((A)(B(C))), for example, there are four such 
groups: 

1 ((A)(B(C))) 2 (A) 3 (B(C)) 4 (C) 

Group zero always stands for the entire expression. 

Capturing groups are so named because, during a match, each subsequence of the 
input sequence that matches such a group is saved. The captured subsequence may 
be used later in the expression, via a back reference, and may also be 
retrieved from the matcher once the match operation is complete. 

The captured input associated with a group is always the subsequence that the 
group most recently matched. If a group is evaluated a second time because of 
quantification then its previously-captured value, if any, will be retained if 
the second evaluation fails. Matching the string "aba" against the expression 
(a(b)?)+, for example, leaves group two set to "b". All captured input is 
discarded at the beginning of each match. 

Groups beginning with (? are pure, non-capturing groups that do not capture 
text and do not count towards the group total. 

Unicode support 

This class is in conformance with Level 1 of Unicode Technical Standard #18: 
Unicode Regular Expression Guidelines, plus RL2.1 Canonical Equivalents. 

Unicode escape sequences such as u2014 in Java source code are processed as 
described in ¤3.3 of the Java Language Specification. Such escape sequences are 
also implemented directly by the regular-expression parser so that Unicode 
escapes can be used in expressions that are read from files or from the 
keyboard. Thus the strings "u2014" and "\\u2014", while not equal, compile into 
the same pattern, which matches the character with hexadecimal value 0x2014. 

Unicode blocks and categories are written with the \p and \P constructs as in 
Perl. \p{prop} matches if the input has the property prop, while \P{prop} does 
not match if the input has that property. Blocks are specified with the prefix 
In, as in InMongolian. Categories may be specified with the optional prefix Is: 
Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and 
categories can be used both inside and outside of a character class. 

The supported categories are those of 

The Unicode Standard in the version specified by the 
Character(|java.lang.Character|) class. The category names are those defined in 
the Standard, both normative and informative. The block names supported by 
Pattern are the valid block names accepted and defined by 
UnicodeBlock.forName(|java.lang.Character.UnicodeBlock|) . 

Categories that behave like the java.lang.Character boolean ismethodname 
methods (except for the deprecated ones) are available through the same 
\p{prop} syntax where the specified property has the name javamethodname. 

Comparison to Perl 5 

The Pattern engine performs traditional NFA-based matching with ordered 
alternation as occurs in Perl 5. 

Perl constructs not supported by this class: 



The conditional constructs (?{X}) and (?(condition)X|Y), 

The embedded code constructs (?{code}) and (??{code}), 

The embedded comment syntax (?#comment), and 

The preprocessing operations \l u, \L, and \U. 



Constructs supported by this class but not by Perl: 



Possessive quantifiers, which greedily match as much as they can and do not 
back off, even when doing so would allow the overall match to succeed. 

Character-class union and intersection as described above. 



Notable differences from Perl: 



In Perl, \1 through \9 are always interpreted as back references; a 
backslash-escaped number greater than 9 is treated as a back reference if at 
least that many subexpressions exist, otherwise it is interpreted, if possible, 
as an octal escape. In this class octal escapes must always begin with a zero. 
In this class, \1 through \9 are always interpreted as back references, and a 
larger number is accepted as a back reference if at least that many 
subexpressions exist at that point in the regular expression, otherwise the 
parser will drop digits until the number is smaller or equal to the existing 
number of groups or it is one digit. 

Perl uses the g flag to request a match that resumes where the last match left 
off. This functionality is provided implicitly by the 
(|java.util.regex.Matcher|) class: Repeated invocations of the 
find(|java.util.regex.Matcher|) method will resume where the last match left 
off, unless the matcher is reset. 

In Perl, embedded flags at the top level of an expression affect the whole 
expression. In this class, embedded flags always take effect at the point at 
which they appear, whether they are at the top level or within a group; in the 
latter case, flags are restored at the end of the group just as in Perl. 

Perl is forgiving about malformed matching constructs, as in the expression *a, 
as well as dangling brackets, as in the expression abc], and treats them as 
literals. This class also accepts dangling brackets but is strict about 
dangling metacharacters like +, ? and *, and will throw a 
(|java.util.regex.PatternSyntaxException|) if it encounters them. 



For a more precise description of the behavior of regular expression 
constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. 
F. Friedl, O'Reilly and Associates, 2002. 



*java.util.regex.Pattern.compile(String)*

public static |java.util.regex.Pattern| compile(java.lang.String regex)

Compiles the given regular expression into a pattern. 

    regex - The expression to be compiled 

*java.util.regex.Pattern.compile(String,int)*

public static |java.util.regex.Pattern| compile(
  java.lang.String regex,
  int flags)

Compiles the given regular expression into a pattern with the given flags. 

    regex - The expression to be compiled 
    flags - Match flags, a bit mask that may include {@link #CASE_INSENSITIVE}, {@link 
       #MULTILINE}, {@link #DOTALL}, {@link #UNICODE_CASE}, and {@link 
       #CANON_EQ} 

*java.util.regex.Pattern.flags()*

public int flags()

Returns this pattern's match flags. 


    Returns: The match flags specified when this pattern was compiled 
*java.util.regex.Pattern.matcher(CharSequence)*

public |java.util.regex.Matcher| matcher(java.lang.CharSequence input)

Creates a matcher that will match the given input against this pattern. 

    input - The character sequence to be matched 

    Returns: A new matcher for this pattern 
*java.util.regex.Pattern.matches(String,CharSequence)*

public static boolean matches(
  java.lang.String regex,
  java.lang.CharSequence input)

Compiles the given regular expression and attempts to match the given input 
against it. 

An invocation of this convenience method of the form 



Pattern.matches(regex, input); 

behaves in exactly the same way as the expression 



Pattern.compile(regex).matcher(input).matches() 

If a pattern is to be used multiple times, compiling it once and reusing it 
will be more efficient than invoking this method each time. 

    regex - The expression to be compiled 
    input - The character sequence to be matched 

*java.util.regex.Pattern.pattern()*

public |java.lang.String| pattern()

Returns the regular expression from which this pattern was compiled. 


    Returns: The source of this pattern 
*java.util.regex.Pattern.quote(String)*

public static |java.lang.String| quote(java.lang.String s)

Returns a literal pattern String for the specified String. 

This method produces a String that can be used to create a Pattern that would 
match the string s as if it were a literal pattern. Metacharacters or escape 
sequences in the input sequence will be given no special meaning. 

    s - The string to be literalized 

    Returns: A literal string replacement 
*java.util.regex.Pattern.split(CharSequence)*

public |java.lang.String| split(java.lang.CharSequence input)

Splits the given input sequence around matches of this pattern. 

This method works as if by invoking the two-argument 
split(|java.util.regex.Pattern|) method with the given input sequence and a 
limit argument of zero. Trailing empty strings are therefore not included in 
the resulting array. 

The input "boo:and:foo", for example, yields the following results with these 
expressions: 

Regex Result : { "boo", "and", "foo" } o { "b", "", ":and:f" } 

    input - The character sequence to be split 

    Returns: The array of strings computed by splitting the input around matches of this 
             pattern 
*java.util.regex.Pattern.split(CharSequence,int)*

public |java.lang.String| split(
  java.lang.CharSequence input,
  int limit)

Splits the given input sequence around matches of this pattern. 

The array returned by this method contains each substring of the input sequence 
that is terminated by another subsequence that matches this pattern or is 
terminated by the end of the input sequence. The substrings in the array are in 
the order in which they occur in the input. If this pattern does not match any 
subsequence of the input then the resulting array has just one element, namely 
the input sequence in string form. 

The limit parameter controls the number of times the pattern is applied and 
therefore affects the length of the resulting array. If the limit n is greater 
than zero then the pattern will be applied at most n-1 times, the array's 
length will be no greater than n, and the array's last entry will contain all 
input beyond the last matched delimiter. If n is non-positive then the pattern 
will be applied as many times as possible and the array can have any length. If 
n is zero then the pattern will be applied as many times as possible, the array 
can have any length, and trailing empty strings will be discarded. 

The input "boo:and:foo", for example, yields the following results with these 
parameters: 

Regex Limit Result : 2 { "boo", "and:foo" } : 5 { "boo", "and", "foo" } : -2 { 
"boo", "and", "foo" } o 5 { "b", "", ":and:f", "", "" } o -2 { "b", "", 
":and:f", "", "" } o 0 { "b", "", ":and:f" } 

    input - The character sequence to be split 
    limit - The result threshold, as described above 

    Returns: The array of strings computed by splitting the input around matches of this 
             pattern 
*java.util.regex.Pattern.toString()*

public |java.lang.String| toString()

Returns the string representation of this pattern. This is the regular 
expression from which this pattern was compiled. 


    Returns: The string representation of this pattern