4 perlop - Perl operators and precedence
8 =head2 Operator Precedence and Associativity
9 X<operator, precedence> X<precedence> X<associativity>
11 Operator precedence and associativity work in Perl more or less like
12 they do in mathematics.
14 I<Operator precedence> means some operators are evaluated before
15 others. For example, in C<2 + 4 * 5>, the multiplication has higher
16 precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
17 22> and not C<6 * 5 == 30>.
19 I<Operator associativity> defines what happens if a sequence of the
20 same operators is used one after another: whether the evaluator will
21 evaluate the left operations first or the right. For example, in C<8
22 - 4 - 2>, subtraction is left associative so Perl evaluates the
23 expression left to right. C<8 - 4> is evaluated first making the
24 expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
26 Perl operators have the following associativity and precedence,
27 listed from highest precedence to lowest. Operators borrowed from
28 C keep the same precedence relationship with each other, even where
29 C's precedence is slightly screwy. (This makes learning Perl easier
30 for C folks.) With very few exceptions, these all operate on scalar
31 values only, not array values.
33 left terms and list operators (leftward)
37 right ! ~ \ and unary + and -
42 nonassoc named unary operators
43 nonassoc < > <= >= lt gt le ge
44 nonassoc == != <=> eq ne cmp
53 nonassoc list operators (rightward)
58 In the following sections, these operators are covered in precedence order.
60 Many operators can be overloaded for objects. See L<overload>.
62 =head2 Terms and List Operators (Leftward)
63 X<list operator> X<operator, list> X<term>
65 A TERM has the highest precedence in Perl. They include variables,
66 quote and quote-like operators, any expression in parentheses,
67 and any function whose arguments are parenthesized. Actually, there
68 aren't really functions in this sense, just list operators and unary
69 operators behaving as functions because you put parentheses around
70 the arguments. These are all documented in L<perlfunc>.
72 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
73 is followed by a left parenthesis as the next token, the operator and
74 arguments within parentheses are taken to be of highest precedence,
75 just like a normal function call.
77 In the absence of parentheses, the precedence of list operators such as
78 C<print>, C<sort>, or C<chmod> is either very high or very low depending on
79 whether you are looking at the left side or the right side of the operator.
82 @ary = (1, 3, sort 4, 2);
83 print @ary; # prints 1324
85 the commas on the right of the sort are evaluated before the sort,
86 but the commas on the left are evaluated after. In other words,
87 list operators tend to gobble up all arguments that follow, and
88 then act like a simple TERM with regard to the preceding expression.
89 Be careful with parentheses:
91 # These evaluate exit before doing the print:
92 print($foo, exit); # Obviously not what you want.
93 print $foo, exit; # Nor is this.
95 # These do the print before evaluating exit:
96 (print $foo), exit; # This is what you want.
97 print($foo), exit; # Or this.
98 print ($foo), exit; # Or even this.
102 print ($foo & 255) + 1, "\n";
104 probably doesn't do what you expect at first glance. The parentheses
105 enclose the argument list for C<print> which is evaluated (printing
106 the result of C<$foo & 255>). Then one is added to the return value
107 of C<print> (usually 1). The result is something like this:
109 1 + 1, "\n"; # Obviously not what you meant.
111 To do what you meant properly, you must write:
113 print(($foo & 255) + 1, "\n");
115 See L<Named Unary Operators> for more discussion of this.
117 Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
118 well as subroutine and method calls, and the anonymous
119 constructors C<[]> and C<{}>.
121 See also L<Quote and Quote-like Operators> toward the end of this section,
122 as well as L<"I/O Operators">.
124 =head2 The Arrow Operator
125 X<arrow> X<dereference> X<< -> >>
127 "C<< -> >>" is an infix dereference operator, just as it is in C
128 and C++. If the right side is either a C<[...]>, C<{...}>, or a
129 C<(...)> subscript, then the left side must be either a hard or
130 symbolic reference to an array, a hash, or a subroutine respectively.
131 (Or technically speaking, a location capable of holding a hard
132 reference, if it's an array or hash reference being used for
133 assignment.) See L<perlreftut> and L<perlref>.
135 Otherwise, the right side is a method name or a simple scalar
136 variable containing either the method name or a subroutine reference,
137 and the left side must be either an object (a blessed reference)
138 or a class name (that is, a package name). See L<perlobj>.
140 =head2 Auto-increment and Auto-decrement
141 X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
143 "++" and "--" work as in C. That is, if placed before a variable,
144 they increment or decrement the variable by one before returning the
145 value, and if placed after, increment or decrement after returning the
149 print $i++; # prints 0
150 print ++$j; # prints 1
152 Note that just as in C, Perl doesn't define B<when> the variable is
153 incremented or decremented. You just know it will be done sometime
154 before or after the value is returned. This also means that modifying
155 a variable twice in the same statement will lead to undefined behaviour.
156 Avoid statements like:
161 Perl will not guarantee what the result of the above statements is.
163 The auto-increment operator has a little extra builtin magic to it. If
164 you increment a variable that is numeric, or that has ever been used in
165 a numeric context, you get a normal increment. If, however, the
166 variable has been used in only string contexts since it was set, and
167 has a value that is not the empty string and matches the pattern
168 C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
169 character within its range, with carry:
171 print ++($foo = '99'); # prints '100'
172 print ++($foo = 'a0'); # prints 'a1'
173 print ++($foo = 'Az'); # prints 'Ba'
174 print ++($foo = 'zz'); # prints 'aaa'
176 C<undef> is always treated as numeric, and in particular is changed
177 to C<0> before incrementing (so that a post-increment of an undef value
178 will return C<0> rather than C<undef>).
180 The auto-decrement operator is not magical.
182 =head2 Exponentiation
183 X<**> X<exponentiation> X<power>
185 Binary "**" is the exponentiation operator. It binds even more
186 tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
187 implemented using C's pow(3) function, which actually works on doubles
190 =head2 Symbolic Unary Operators
191 X<unary operator> X<operator, unary>
193 Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
194 precedence version of this.
197 Unary "-" performs arithmetic negation if the operand is numeric. If
198 the operand is an identifier, a string consisting of a minus sign
199 concatenated with the identifier is returned. Otherwise, if the string
200 starts with a plus or minus, a string starting with the opposite sign
201 is returned. One effect of these rules is that -bareword is equivalent
202 to the string "-bareword". If, however, the string begins with a
203 non-alphabetic character (exluding "+" or "-"), Perl will attempt to convert
204 the string to a numeric and the arithmetic negation is performed. If the
205 string cannot be cleanly converted to a numeric, Perl will give the warning
206 B<Argument "the string" isn't numeric in negation (-) at ...>.
207 X<-> X<negation, arithmetic>
209 Unary "~" performs bitwise negation, i.e., 1's complement. For
210 example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
211 L<Bitwise String Operators>.) Note that the width of the result is
212 platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
213 bits wide on a 64-bit platform, so if you are expecting a certain bit
214 width, remember to use the & operator to mask off the excess bits.
215 X<~> X<negation, binary>
217 Unary "+" has no effect whatsoever, even on strings. It is useful
218 syntactically for separating a function name from a parenthesized expression
219 that would otherwise be interpreted as the complete list of function
220 arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
223 Unary "\" creates a reference to whatever follows it. See L<perlreftut>
224 and L<perlref>. Do not confuse this behavior with the behavior of
225 backslash within a string, although both forms do convey the notion
226 of protecting the next thing from interpolation.
227 X<\> X<reference> X<backslash>
229 =head2 Binding Operators
230 X<binding> X<operator, binding> X<=~> X<!~>
232 Binary "=~" binds a scalar expression to a pattern match. Certain operations
233 search or modify the string $_ by default. This operator makes that kind
234 of operation work on some other string. The right argument is a search
235 pattern, substitution, or transliteration. The left argument is what is
236 supposed to be searched, substituted, or transliterated instead of the default
237 $_. When used in scalar context, the return value generally indicates the
238 success of the operation. Behavior in list context depends on the particular
239 operator. See L</"Regexp Quote-Like Operators"> for details and
240 L<perlretut> for examples using these operators.
242 If the right argument is an expression rather than a search pattern,
243 substitution, or transliteration, it is interpreted as a search pattern at run
246 Binary "!~" is just like "=~" except the return value is negated in
249 =head2 Multiplicative Operators
250 X<operator, multiplicative>
252 Binary "*" multiplies two numbers.
255 Binary "/" divides two numbers.
258 Binary "%" computes the modulus of two numbers. Given integer
259 operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
260 C<$a> minus the largest multiple of C<$b> that is not greater than
261 C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
262 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
263 result will be less than or equal to zero).
264 Note that when C<use integer> is in scope, "%" gives you direct access
265 to the modulus operator as implemented by your C compiler. This
266 operator is not as well defined for negative operands, but it will
268 X<%> X<remainder> X<modulus> X<mod>
270 Binary "x" is the repetition operator. In scalar context or if the left
271 operand is not enclosed in parentheses, it returns a string consisting
272 of the left operand repeated the number of times specified by the right
273 operand. In list context, if the left operand is enclosed in
274 parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
275 If the right operand is zero or negative, it returns an empty string
276 or an empty list, depending on the context.
279 print '-' x 80; # print row of dashes
281 print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
283 @ones = (1) x 80; # a list of 80 1's
284 @ones = (5) x @ones; # set all elements to 5
287 =head2 Additive Operators
288 X<operator, additive>
290 Binary "+" returns the sum of two numbers.
293 Binary "-" returns the difference of two numbers.
296 Binary "." concatenates two strings.
297 X<string, concatenation> X<concatenation>
298 X<cat> X<concat> X<concatenate> X<.>
300 =head2 Shift Operators
301 X<shift operator> X<operator, shift> X<<< << >>>
302 X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
303 X<shl> X<shr> X<shift, right> X<shift, left>
305 Binary "<<" returns the value of its left argument shifted left by the
306 number of bits specified by the right argument. Arguments should be
307 integers. (See also L<Integer Arithmetic>.)
309 Binary ">>" returns the value of its left argument shifted right by
310 the number of bits specified by the right argument. Arguments should
311 be integers. (See also L<Integer Arithmetic>.)
313 Note that both "<<" and ">>" in Perl are implemented directly using
314 "<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
315 in force then signed C integers are used, else unsigned C integers are
316 used. Either way, the implementation isn't going to generate results
317 larger than the size of the integer type Perl was built with (32 bits
320 The result of overflowing the range of the integers is undefined
321 because it is undefined also in C. In other words, using 32-bit
322 integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
323 of bits is also undefined.
325 =head2 Named Unary Operators
326 X<operator, named unary>
328 The various named unary operators are treated as functions with one
329 argument, with optional parentheses.
331 If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
332 is followed by a left parenthesis as the next token, the operator and
333 arguments within parentheses are taken to be of highest precedence,
334 just like a normal function call. For example,
335 because named unary operators are higher precedence than ||:
337 chdir $foo || die; # (chdir $foo) || die
338 chdir($foo) || die; # (chdir $foo) || die
339 chdir ($foo) || die; # (chdir $foo) || die
340 chdir +($foo) || die; # (chdir $foo) || die
342 but, because * is higher precedence than named operators:
344 chdir $foo * 20; # chdir ($foo * 20)
345 chdir($foo) * 20; # (chdir $foo) * 20
346 chdir ($foo) * 20; # (chdir $foo) * 20
347 chdir +($foo) * 20; # chdir ($foo * 20)
349 rand 10 * 20; # rand (10 * 20)
350 rand(10) * 20; # (rand 10) * 20
351 rand (10) * 20; # (rand 10) * 20
352 rand +(10) * 20; # rand (10 * 20)
354 Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
355 treated like named unary operators, but they don't follow this functional
356 parenthesis rule. That means, for example, that C<-f($file).".bak"> is
357 equivalent to C<-f "$file.bak">.
358 X<-X> X<filetest> X<operator, filetest>
360 See also L<"Terms and List Operators (Leftward)">.
362 =head2 Relational Operators
363 X<relational operator> X<operator, relational>
365 Binary "<" returns true if the left argument is numerically less than
369 Binary ">" returns true if the left argument is numerically greater
370 than the right argument.
373 Binary "<=" returns true if the left argument is numerically less than
374 or equal to the right argument.
377 Binary ">=" returns true if the left argument is numerically greater
378 than or equal to the right argument.
381 Binary "lt" returns true if the left argument is stringwise less than
385 Binary "gt" returns true if the left argument is stringwise greater
386 than the right argument.
389 Binary "le" returns true if the left argument is stringwise less than
390 or equal to the right argument.
393 Binary "ge" returns true if the left argument is stringwise greater
394 than or equal to the right argument.
397 =head2 Equality Operators
398 X<equality> X<equal> X<equals> X<operator, equality>
400 Binary "==" returns true if the left argument is numerically equal to
404 Binary "!=" returns true if the left argument is numerically not equal
405 to the right argument.
408 Binary "<=>" returns -1, 0, or 1 depending on whether the left
409 argument is numerically less than, equal to, or greater than the right
410 argument. If your platform supports NaNs (not-a-numbers) as numeric
411 values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
412 "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
413 returns true, as does NaN != anything else. If your platform doesn't
414 support NaNs then NaN is just a string with numeric value 0.
415 X<< <=> >> X<spaceship>
417 perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
418 perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
420 Binary "eq" returns true if the left argument is stringwise equal to
424 Binary "ne" returns true if the left argument is stringwise not equal
425 to the right argument.
428 Binary "cmp" returns -1, 0, or 1 depending on whether the left
429 argument is stringwise less than, equal to, or greater than the right
433 "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
434 by the current locale if C<use locale> is in effect. See L<perllocale>.
437 X<operator, bitwise, and> X<bitwise and> X<&>
439 Binary "&" returns its operands ANDed together bit by bit.
440 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
442 Note that "&" has lower priority than relational operators, so for example
443 the brackets are essential in a test like
445 print "Even\n" if ($x & 1) == 0;
447 =head2 Bitwise Or and Exclusive Or
448 X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
451 Binary "|" returns its operands ORed together bit by bit.
452 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
454 Binary "^" returns its operands XORed together bit by bit.
455 (See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
457 Note that "|" and "^" have lower priority than relational operators, so
458 for example the brackets are essential in a test like
460 print "false\n" if (8 | 2) != 10;
462 =head2 C-style Logical And
463 X<&&> X<logical and> X<operator, logical, and>
465 Binary "&&" performs a short-circuit logical AND operation. That is,
466 if the left operand is false, the right operand is not even evaluated.
467 Scalar or list context propagates down to the right operand if it
470 =head2 C-style Logical Or
471 X<||> X<operator, logical, or>
473 Binary "||" performs a short-circuit logical OR operation. That is,
474 if the left operand is true, the right operand is not even evaluated.
475 Scalar or list context propagates down to the right operand if it
478 The C<||> and C<&&> operators return the last value evaluated
479 (unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
480 portable way to find out the home directory might be:
482 $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
483 (getpwuid($<))[7] || die "You're homeless!\n";
485 In particular, this means that you shouldn't use this
486 for selecting between two aggregates for assignment:
488 @a = @b || @c; # this is wrong
489 @a = scalar(@b) || @c; # really meant this
490 @a = @b ? @b : @c; # this works fine, though
492 As more readable alternatives to C<&&> and C<||> when used for
493 control flow, Perl provides C<and> and C<or> operators (see below).
494 The short-circuit behavior is identical. The precedence of "and" and
495 "or" is much lower, however, so that you can safely use them after a
496 list operator without the need for parentheses:
498 unlink "alpha", "beta", "gamma"
499 or gripe(), next LINE;
501 With the C-style operators that would have been written like this:
503 unlink("alpha", "beta", "gamma")
504 || (gripe(), next LINE);
506 Using "or" for assignment is unlikely to do what you want; see below.
508 =head2 Range Operators
509 X<operator, range> X<range> X<..> X<...>
511 Binary ".." is the range operator, which is really two different
512 operators depending on the context. In list context, it returns a
513 list of values counting (up by ones) from the left value to the right
514 value. If the left value is greater than the right value then it
515 returns the empty list. The range operator is useful for writing
516 C<foreach (1..10)> loops and for doing slice operations on arrays. In
517 the current implementation, no temporary array is created when the
518 range operator is used as the expression in C<foreach> loops, but older
519 versions of Perl might burn a lot of memory when you write something
522 for (1 .. 1_000_000) {
526 The range operator also works on strings, using the magical auto-increment,
529 In scalar context, ".." returns a boolean value. The operator is
530 bistable, like a flip-flop, and emulates the line-range (comma) operator
531 of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
532 own boolean state. It is false as long as its left operand is false.
533 Once the left operand is true, the range operator stays true until the
534 right operand is true, I<AFTER> which the range operator becomes false
535 again. It doesn't become false till the next time the range operator is
536 evaluated. It can test the right operand and become false on the same
537 evaluation it became true (as in B<awk>), but it still returns true once.
538 If you don't want it to test the right operand till the next
539 evaluation, as in B<sed>, just use three dots ("...") instead of
540 two. In all other regards, "..." behaves just like ".." does.
542 The right operand is not evaluated while the operator is in the
543 "false" state, and the left operand is not evaluated while the
544 operator is in the "true" state. The precedence is a little lower
545 than || and &&. The value returned is either the empty string for
546 false, or a sequence number (beginning with 1) for true. The
547 sequence number is reset for each range encountered. The final
548 sequence number in a range has the string "E0" appended to it, which
549 doesn't affect its numeric value, but gives you something to search
550 for if you want to exclude the endpoint. You can exclude the
551 beginning point by waiting for the sequence number to be greater
554 If either operand of scalar ".." is a constant expression,
555 that operand is considered true if it is equal (C<==>) to the current
556 input line number (the C<$.> variable).
558 To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
559 but that is only an issue if you use a floating point expression; when
560 implicitly using C<$.> as described in the previous paragraph, the
561 comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
562 is set to a floating point value and you are not reading from a file.
563 Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
564 you want in scalar context because each of the operands are evaluated
565 using their integer representation.
569 As a scalar operator:
571 if (101 .. 200) { print; } # print 2nd hundred lines, short for
572 # if ($. == 101 .. $. == 200) ...
574 next LINE if (1 .. /^$/); # skip header lines, short for
575 # ... if ($. == 1 .. /^$/);
576 # (typically in a loop labeled LINE)
578 s/^/> / if (/^$/ .. eof()); # quote body
580 # parse mail messages
582 $in_header = 1 .. /^$/;
583 $in_body = /^$/ .. eof;
590 close ARGV if eof; # reset $. each file
593 Here's a simple example to illustrate the difference between
594 the two range operators:
607 This program will print only the line containing "Bar". If
608 the range operator is changed to C<...>, it will also print the
611 And now some examples as a list operator:
613 for (101 .. 200) { print; } # print $_ 100 times
614 @foo = @foo[0 .. $#foo]; # an expensive no-op
615 @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
617 The range operator (in list context) makes use of the magical
618 auto-increment algorithm if the operands are strings. You
621 @alphabet = ('A' .. 'Z');
623 to get all normal letters of the English alphabet, or
625 $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
627 to get a hexadecimal digit, or
629 @z2 = ('01' .. '31'); print $z2[$mday];
631 to get dates with leading zeros. If the final value specified is not
632 in the sequence that the magical increment would produce, the sequence
633 goes until the next value would be longer than the final value
636 Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
637 return two elements in list context.
639 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
641 =head2 Conditional Operator
642 X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
644 Ternary "?:" is the conditional operator, just as in C. It works much
645 like an if-then-else. If the argument before the ? is true, the
646 argument before the : is returned, otherwise the argument after the :
647 is returned. For example:
649 printf "I have %d dog%s.\n", $n,
650 ($n == 1) ? '' : "s";
652 Scalar or list context propagates downward into the 2nd
653 or 3rd argument, whichever is selected.
655 $a = $ok ? $b : $c; # get a scalar
656 @a = $ok ? @b : @c; # get an array
657 $a = $ok ? @b : @c; # oops, that's just a count!
659 The operator may be assigned to if both the 2nd and 3rd arguments are
660 legal lvalues (meaning that you can assign to them):
662 ($a_or_b ? $a : $b) = $c;
664 Because this operator produces an assignable result, using assignments
665 without parentheses will get you in trouble. For example, this:
667 $a % 2 ? $a += 10 : $a += 2
671 (($a % 2) ? ($a += 10) : $a) += 2
675 ($a % 2) ? ($a += 10) : ($a += 2)
677 That should probably be written more simply as:
679 $a += ($a % 2) ? 10 : 2;
681 =head2 Assignment Operators
682 X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
683 X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<.=>
686 "=" is the ordinary assignment operator.
688 Assignment operators work as in C. That is,
696 although without duplicating any side effects that dereferencing the lvalue
697 might trigger, such as from tie(). Other assignment operators work similarly.
698 The following are recognized:
705 Although these are grouped by family, they all have the precedence
708 Unlike in C, the scalar assignment operator produces a valid lvalue.
709 Modifying an assignment is equivalent to doing the assignment and
710 then modifying the variable that was assigned to. This is useful
711 for modifying a copy of something, like this:
713 ($tmp = $global) =~ tr [A-Z] [a-z];
724 Similarly, a list assignment in list context produces the list of
725 lvalues assigned to, and a list assignment in scalar context returns
726 the number of elements produced by the expression on the right hand
727 side of the assignment.
729 =head2 Comma Operator
730 X<comma> X<operator, comma> X<,>
732 Binary "," is the comma operator. In scalar context it evaluates
733 its left argument, throws that value away, then evaluates its right
734 argument and returns that value. This is just like C's comma operator.
736 In list context, it's just the list argument separator, and inserts
737 both its arguments into the list.
739 The C<< => >> operator is a synonym for the comma, but forces any word
740 (consisting entirely of word characters) to its left to be interpreted
741 as a string (as of 5.001). This includes words that might otherwise be
742 considered a constant or function call.
744 use constant FOO => "something";
746 my %h = ( FOO => 23 );
754 my %h = ("something", 23);
756 If the argument on the left is not a word, it is first interpreted as
757 an expression, and then the string value of that is used.
759 The C<< => >> operator is helpful in documenting the correspondence
760 between keys and values in hashes, and other paired elements in lists.
762 %hash = ( $key => $value );
763 login( $username => $password );
765 =head2 List Operators (Rightward)
766 X<operator, list, rightward> X<list operator>
768 On the right side of a list operator, it has very low precedence,
769 such that it controls all comma-separated expressions found there.
770 The only operators with lower precedence are the logical operators
771 "and", "or", and "not", which may be used to evaluate calls to list
772 operators without the need for extra parentheses:
774 open HANDLE, "filename"
775 or die "Can't open: $!\n";
777 See also discussion of list operators in L<Terms and List Operators (Leftward)>.
780 X<operator, logical, not> X<not>
782 Unary "not" returns the logical negation of the expression to its right.
783 It's the equivalent of "!" except for the very low precedence.
786 X<operator, logical, and> X<and>
788 Binary "and" returns the logical conjunction of the two surrounding
789 expressions. It's equivalent to && except for the very low
790 precedence. This means that it short-circuits: i.e., the right
791 expression is evaluated only if the left expression is true.
793 =head2 Logical or and Exclusive Or
794 X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
795 X<operator, logical, defined or> X<operator, logical, exclusive or>
798 Binary "or" returns the logical disjunction of the two surrounding
799 expressions. It's equivalent to || except for the very low precedence.
800 This makes it useful for control flow
802 print FH $data or die "Can't write to FH: $!";
804 This means that it short-circuits: i.e., the right expression is evaluated
805 only if the left expression is false. Due to its precedence, you should
806 probably avoid using this for assignment, only for control flow.
808 $a = $b or $c; # bug: this is wrong
809 ($a = $b) or $c; # really means this
810 $a = $b || $c; # better written this way
812 However, when it's a list-context assignment and you're trying to use
813 "||" for control flow, you probably need "or" so that the assignment
814 takes higher precedence.
816 @info = stat($file) || die; # oops, scalar sense of stat!
817 @info = stat($file) or die; # better, now @info gets its due
819 Then again, you could always use parentheses.
821 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
822 It cannot short circuit, of course.
824 =head2 C Operators Missing From Perl
825 X<operator, missing from perl> X<&> X<*>
826 X<typecasting> X<(TYPE)>
828 Here is what C has that Perl doesn't:
834 Address-of operator. (But see the "\" operator for taking a reference.)
838 Dereference-address operator. (Perl's prefix dereferencing
839 operators are typed: $, @, %, and &.)
843 Type-casting operator.
847 =head2 Quote and Quote-like Operators
848 X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
849 X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
850 X<escape sequence> X<escape>
853 While we usually think of quotes as literal values, in Perl they
854 function as operators, providing various kinds of interpolating and
855 pattern matching capabilities. Perl provides customary quote characters
856 for these behaviors, but also provides a way for you to choose your
857 quote character for any of them. In the following table, a C<{}> represents
858 any pair of delimiters you choose.
860 Customary Generic Meaning Interpolates
865 // m{} Pattern match yes*
867 s{}{} Substitution yes*
868 tr{}{} Transliteration no (but see below)
871 * unless the delimiter is ''.
873 Non-bracketing delimiters use the same character fore and aft, but the four
874 sorts of brackets (round, angle, square, curly) will all nest, which means
883 Note, however, that this does not always work for quoting Perl code:
885 $s = q{ if($a eq "}") ... }; # WRONG
887 is a syntax error. The C<Text::Balanced> module (from CPAN, and
888 starting from Perl 5.8 part of the standard distribution) is able
891 There can be whitespace between the operator and the quoting
892 characters, except when C<#> is being used as the quoting character.
893 C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
894 operator C<q> followed by a comment. Its argument will be taken
895 from the next line. This allows you to write:
897 s {foo} # Replace foo
900 The following escape sequences are available in constructs that interpolate
901 and in transliterations.
902 X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N>
909 \a alarm (bell) (BEL)
911 \033 octal char (ESC)
913 \x{263a} wide hex char (SMILEY)
914 \c[ control char (ESC)
915 \N{name} named Unicode character
917 B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
918 the vertical tab (VT - ASCII 11).
920 The following escape sequences are available in constructs that interpolate
921 but not in transliterations.
922 X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
924 \l lowercase next char
925 \u uppercase next char
928 \E end case modification
929 \Q quote non-word characters till \E
931 If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
932 C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
933 If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
934 beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
935 C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
938 All systems use the virtual C<"\n"> to represent a line terminator,
939 called a "newline". There is no such thing as an unvarying, physical
940 newline character. It is only an illusion that the operating system,
941 device drivers, C libraries, and Perl all conspire to preserve. Not all
942 systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
943 on a Mac, these are reversed, and on systems without line terminator,
944 printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
945 you mean a "newline" for your system, but use the literal ASCII when you
946 need an exact character. For example, most networking protocols expect
947 and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
948 and although they often accept just C<"\012">, they seldom tolerate just
949 C<"\015">. If you get in the habit of using C<"\n"> for networking,
950 you may be burned some day.
951 X<newline> X<line terminator> X<eol> X<end of line>
954 For constructs that do interpolate, variables beginning with "C<$>"
955 or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
956 C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
957 But method calls such as C<< $obj->meth >> are not.
959 Interpolating an array or slice interpolates the elements in order,
960 separated by the value of C<$">, so is equivalent to interpolating
961 C<join $", @array>. "Punctuation" arrays such as C<@+> are only
962 interpolated if the name is enclosed in braces C<@{+}>.
964 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
965 An unescaped C<$> or C<@> interpolates the corresponding variable,
966 while escaping will cause the literal string C<\$> to be inserted.
967 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
969 Patterns are subject to an additional level of interpretation as a
970 regular expression. This is done as a second pass, after variables are
971 interpolated, so that regular expressions may be incorporated into the
972 pattern from the variables. If this is not what you want, use C<\Q> to
973 interpolate a variable literally.
975 Apart from the behavior described above, Perl does not expand
976 multiple levels of interpolation. In particular, contrary to the
977 expectations of shell programmers, back-quotes do I<NOT> interpolate
978 within double quotes, nor do single quotes impede evaluation of
979 variables when used within double quotes.
981 =head2 Regexp Quote-Like Operators
984 Here are the quote-like operators that apply to pattern
985 matching and related activities.
992 This is just like the C</pattern/> search, except that it matches only
993 once between calls to the reset() operator. This is a useful
994 optimization when you want to see only the first occurrence of
995 something in each file of a set of files, for instance. Only C<??>
996 patterns local to the current package are reset.
1000 # blank line between header and body
1003 reset if eof; # clear ?? status for next file
1006 This usage is vaguely deprecated, which means it just might possibly
1007 be removed in some distant future version of Perl, perhaps somewhere
1008 around the year 2168.
1010 =item m/PATTERN/cgimosx
1011 X<m> X<operator, match>
1012 X<regexp, options> X<regexp> X<regex, options> X<regex>
1013 X</c> X</i> X</m> X</o> X</s> X</x>
1015 =item /PATTERN/cgimosx
1017 Searches a string for a pattern match, and in scalar context returns
1018 true if it succeeds, false if it fails. If no string is specified
1019 via the C<=~> or C<!~> operator, the $_ string is searched. (The
1020 string specified with C<=~> need not be an lvalue--it may be the
1021 result of an expression evaluation, but remember the C<=~> binds
1022 rather tightly.) See also L<perlre>. See L<perllocale> for
1023 discussion of additional considerations that apply when C<use locale>
1028 c Do not reset search position on a failed match when /g is in effect.
1029 g Match globally, i.e., find all occurrences.
1030 i Do case-insensitive pattern matching.
1031 m Treat string as multiple lines.
1032 o Compile pattern only once.
1033 s Treat string as single line.
1034 x Use extended regular expressions.
1036 If "/" is the delimiter then the initial C<m> is optional. With the C<m>
1037 you can use any pair of non-alphanumeric, non-whitespace characters
1038 as delimiters. This is particularly useful for matching path names
1039 that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
1040 the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
1041 If "'" is the delimiter, no interpolation is performed on the PATTERN.
1043 PATTERN may contain variables, which will be interpolated (and the
1044 pattern recompiled) every time the pattern search is evaluated, except
1045 for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
1046 C<$|> are not interpolated because they look like end-of-string tests.)
1047 If you want such a pattern to be compiled only once, add a C</o> after
1048 the trailing delimiter. This avoids expensive run-time recompilations,
1049 and is useful when the value you are interpolating won't change over
1050 the life of the script. However, mentioning C</o> constitutes a promise
1051 that you won't change the variables in the pattern. If you change them,
1052 Perl won't even notice. See also L<"qr/STRING/imosx">.
1054 If the PATTERN evaluates to the empty string, the last
1055 I<successfully> matched regular expression is used instead. In this
1056 case, only the C<g> and C<c> flags on the empty pattern is honoured -
1057 the other flags are taken from the original pattern. If no match has
1058 previously succeeded, this will (silently) act instead as a genuine
1059 empty pattern (which will always match).
1061 If the C</g> option is not used, C<m//> in list context returns a
1062 list consisting of the subexpressions matched by the parentheses in the
1063 pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
1064 also set, and that this differs from Perl 4's behavior.) When there are
1065 no parentheses in the pattern, the return value is the list C<(1)> for
1066 success. With or without parentheses, an empty list is returned upon
1071 open(TTY, '/dev/tty');
1072 <TTY> =~ /^y/i && foo(); # do foo if desired
1074 if (/Version: *([0-9.]*)/) { $version = $1; }
1076 next if m#^/usr/spool/uucp#;
1081 print if /$arg/o; # compile only once
1084 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
1086 This last example splits $foo into the first two words and the
1087 remainder of the line, and assigns those three fields to $F1, $F2, and
1088 $Etc. The conditional is true if any variables were assigned, i.e., if
1089 the pattern matched.
1091 The C</g> modifier specifies global pattern matching--that is,
1092 matching as many times as possible within the string. How it behaves
1093 depends on the context. In list context, it returns a list of the
1094 substrings matched by any capturing parentheses in the regular
1095 expression. If there are no parentheses, it returns a list of all
1096 the matched strings, as if there were parentheses around the whole
1099 In scalar context, each execution of C<m//g> finds the next match,
1100 returning true if it matches, and false if there is no further match.
1101 The position after the last match can be read or set using the pos()
1102 function; see L<perlfunc/pos>. A failed match normally resets the
1103 search position to the beginning of the string, but you can avoid that
1104 by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
1105 string also resets the search position.
1107 You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
1108 zero-width assertion that matches the exact position where the previous
1109 C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
1110 still anchors at pos(), but the match is of course only attempted once.
1111 Using C<\G> without C</g> on a target string that has not previously had a
1112 C</g> match applied to it is the same as using the C<\A> assertion to match
1113 the beginning of the string. Note also that, currently, C<\G> is only
1114 properly supported when anchored at the very beginning of the pattern.
1119 ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
1123 while (defined($paragraph = <>)) {
1124 while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
1128 print "$sentences\n";
1130 # using m//gc with \G
1134 print $1 while /(o)/gc; print "', pos=", pos, "\n";
1136 print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
1138 print $1 while /(p)/gc; print "', pos=", pos, "\n";
1140 print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
1142 The last example should print:
1152 Notice that the final match matched C<q> instead of C<p>, which a match
1153 without the C<\G> anchor would have done. Also note that the final match
1154 did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
1155 final match did indeed match C<p>, it's a good bet that you're running an
1156 older (pre-5.6.0) Perl.
1158 A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
1159 combine several regexps like this to process a string part-by-part,
1160 doing different actions depending on which regexp matched. Each
1161 regexp tries to match where the previous one leaves off.
1164 $url = new URI::URL "http://www/"; die if $url eq "xXx";
1168 print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
1169 print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
1170 print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
1171 print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
1172 print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
1173 print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
1174 print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
1175 print ". That's all!\n";
1178 Here is the output (split into several lines):
1180 line-noise lowercase line-noise lowercase UPPERCASE line-noise
1181 UPPERCASE line-noise lowercase line-noise lowercase line-noise
1182 lowercase lowercase line-noise lowercase lowercase line-noise
1183 MiXeD line-noise. That's all!
1186 X<q> X<quote, double> X<'> X<''>
1190 A single-quoted, literal string. A backslash represents a backslash
1191 unless followed by the delimiter or another backslash, in which case
1192 the delimiter or backslash is interpolated.
1194 $foo = q!I said, "You said, 'She said it.'"!;
1195 $bar = q('This is it.');
1196 $baz = '\n'; # a two-character string
1199 X<qq> X<quote, double> X<"> X<"">
1203 A double-quoted, interpolated string.
1206 (*** The previous line contains the naughty word "$1".\n)
1207 if /\b(tcl|java|python)\b/i; # :-)
1208 $baz = "\n"; # a one-character string
1210 =item qr/STRING/imosx
1211 X<qr> X</i> X</m> X</o> X</s> X</x>
1213 This operator quotes (and possibly compiles) its I<STRING> as a regular
1214 expression. I<STRING> is interpolated the same way as I<PATTERN>
1215 in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
1216 is done. Returns a Perl value which may be used instead of the
1217 corresponding C</STRING/imosx> expression.
1221 $rex = qr/my.STRING/is;
1228 The result may be used as a subpattern in a match:
1231 $string =~ /foo${re}bar/; # can be interpolated in other patterns
1232 $string =~ $re; # or used standalone
1233 $string =~ /$re/; # or this way
1235 Since Perl may compile the pattern at the moment of execution of qr()
1236 operator, using qr() may have speed advantages in some situations,
1237 notably if the result of qr() is used standalone:
1240 my $patterns = shift;
1241 my @compiled = map qr/$_/i, @$patterns;
1244 foreach my $pat (@compiled) {
1245 $success = 1, last if /$pat/;
1251 Precompilation of the pattern into an internal representation at
1252 the moment of qr() avoids a need to recompile the pattern every
1253 time a match C</$pat/> is attempted. (Perl has many other internal
1254 optimizations, but none would be triggered in the above example if
1255 we did not use qr() operator.)
1259 i Do case-insensitive pattern matching.
1260 m Treat string as multiple lines.
1261 o Compile pattern only once.
1262 s Treat string as single line.
1263 x Use extended regular expressions.
1265 See L<perlre> for additional information on valid syntax for STRING, and
1266 for a detailed look at the semantics of regular expressions.
1269 X<qx> X<`> X<``> X<backtick>
1273 A string which is (possibly) interpolated and then executed as a
1274 system command with C</bin/sh> or its equivalent. Shell wildcards,
1275 pipes, and redirections will be honored. The collected standard
1276 output of the command is returned; standard error is unaffected. In
1277 scalar context, it comes back as a single (potentially multi-line)
1278 string, or undef if the command failed. In list context, returns a
1279 list of lines (however you've defined lines with $/ or
1280 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
1282 Because backticks do not affect standard error, use shell file descriptor
1283 syntax (assuming the shell supports this) if you care to address this.
1284 To capture a command's STDERR and STDOUT together:
1286 $output = `cmd 2>&1`;
1288 To capture a command's STDOUT but discard its STDERR:
1290 $output = `cmd 2>/dev/null`;
1292 To capture a command's STDERR but discard its STDOUT (ordering is
1295 $output = `cmd 2>&1 1>/dev/null`;
1297 To exchange a command's STDOUT and STDERR in order to capture the STDERR
1298 but leave its STDOUT to come out the old STDERR:
1300 $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
1302 To read both a command's STDOUT and its STDERR separately, it's easiest
1303 to redirect them separately to files, and then read from those files
1304 when the program is done:
1306 system("program args 1>program.stdout 2>program.stderr");
1308 Using single-quote as a delimiter protects the command from Perl's
1309 double-quote interpolation, passing it on to the shell instead:
1311 $perl_info = qx(ps $$); # that's Perl's $$
1312 $shell_info = qx'ps $$'; # that's the new shell's $$
1314 How that string gets evaluated is entirely subject to the command
1315 interpreter on your system. On most platforms, you will have to protect
1316 shell metacharacters if you want them treated literally. This is in
1317 practice difficult to do, as it's unclear how to escape which characters.
1318 See L<perlsec> for a clean and safe example of a manual fork() and exec()
1319 to emulate backticks safely.
1321 On some platforms (notably DOS-like ones), the shell may not be
1322 capable of dealing with multiline commands, so putting newlines in
1323 the string may not get you what you want. You may be able to evaluate
1324 multiple commands in a single line by separating them with the command
1325 separator character, if your shell supports that (e.g. C<;> on many Unix
1326 shells; C<&> on the Windows NT C<cmd> shell).
1328 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1329 output before starting the child process, but this may not be supported
1330 on some platforms (see L<perlport>). To be safe, you may need to set
1331 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1332 C<IO::Handle> on any open handles.
1334 Beware that some command shells may place restrictions on the length
1335 of the command line. You must ensure your strings don't exceed this
1336 limit after any necessary interpolations. See the platform-specific
1337 release notes for more details about your particular environment.
1339 Using this operator can lead to programs that are difficult to port,
1340 because the shell commands called vary between systems, and may in
1341 fact not be present at all. As one example, the C<type> command under
1342 the POSIX shell is very different from the C<type> command under DOS.
1343 That doesn't mean you should go out of your way to avoid backticks
1344 when they're the right way to get something done. Perl was made to be
1345 a glue language, and one of the things it glues together is commands.
1346 Just understand what you're getting yourself into.
1348 See L<"I/O Operators"> for more discussion.
1351 X<qw> X<quote, list> X<quote, words>
1353 Evaluates to a list of the words extracted out of STRING, using embedded
1354 whitespace as the word delimiters. It can be understood as being roughly
1357 split(' ', q/STRING/);
1359 the differences being that it generates a real list at compile time, and
1360 in scalar context it returns the last element in the list. So
1365 is semantically equivalent to the list:
1369 Some frequently seen examples:
1371 use POSIX qw( setlocale localeconv )
1372 @EXPORT = qw( foo bar baz );
1374 A common mistake is to try to separate the words with comma or to
1375 put comments into a multi-line C<qw>-string. For this reason, the
1376 C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
1377 produces warnings if the STRING contains the "," or the "#" character.
1379 =item s/PATTERN/REPLACEMENT/egimosx
1380 X<substitute> X<substitution> X<replace> X<regexp, replace>
1381 X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
1383 Searches a string for a pattern, and if found, replaces that pattern
1384 with the replacement text and returns the number of substitutions
1385 made. Otherwise it returns false (specifically, the empty string).
1387 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
1388 variable is searched and modified. (The string specified with C<=~> must
1389 be scalar variable, an array element, a hash element, or an assignment
1390 to one of those, i.e., an lvalue.)
1392 If the delimiter chosen is a single quote, no interpolation is
1393 done on either the PATTERN or the REPLACEMENT. Otherwise, if the
1394 PATTERN contains a $ that looks like a variable rather than an
1395 end-of-string test, the variable will be interpolated into the pattern
1396 at run-time. If you want the pattern compiled only once the first time
1397 the variable is interpolated, use the C</o> option. If the pattern
1398 evaluates to the empty string, the last successfully executed regular
1399 expression is used instead. See L<perlre> for further explanation on these.
1400 See L<perllocale> for discussion of additional considerations that apply
1401 when C<use locale> is in effect.
1405 e Evaluate the right side as an expression.
1406 g Replace globally, i.e., all occurrences.
1407 i Do case-insensitive pattern matching.
1408 m Treat string as multiple lines.
1409 o Compile pattern only once.
1410 s Treat string as single line.
1411 x Use extended regular expressions.
1413 Any non-alphanumeric, non-whitespace delimiter may replace the
1414 slashes. If single quotes are used, no interpretation is done on the
1415 replacement string (the C</e> modifier overrides this, however). Unlike
1416 Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
1417 text is not evaluated as a command. If the
1418 PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
1419 pair of quotes, which may or may not be bracketing quotes, e.g.,
1420 C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
1421 replacement portion to be treated as a full-fledged Perl expression
1422 and evaluated right then and there. It is, however, syntax checked at
1423 compile-time. A second C<e> modifier will cause the replacement portion
1424 to be C<eval>ed before being run as a Perl expression.
1428 s/\bgreen\b/mauve/g; # don't change wintergreen
1430 $path =~ s|/usr/bin|/usr/local/bin|;
1432 s/Login: $foo/Login: $bar/; # run-time pattern
1434 ($foo = $bar) =~ s/this/that/; # copy first, then change
1436 $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
1439 s/\d+/$&*2/e; # yields 'abc246xyz'
1440 s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
1441 s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
1443 s/%(.)/$percent{$1}/g; # change percent escapes; no /e
1444 s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
1445 s/^=(\w+)/&pod($1)/ge; # use function call
1447 # expand variables in $_, but dynamics only, using
1448 # symbolic dereferencing
1451 # Add one to the value of any numbers in the string
1454 # This will expand any embedded scalar variable
1455 # (including lexicals) in $_ : First $1 is interpolated
1456 # to the variable name, and then evaluated
1459 # Delete (most) C comments.
1461 /\* # Match the opening delimiter.
1462 .*? # Match a minimal number of characters.
1463 \*/ # Match the closing delimiter.
1466 s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
1468 for ($variable) { # trim whitespace in $variable, cheap
1473 s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
1475 Note the use of $ instead of \ in the last example. Unlike
1476 B<sed>, we use the \<I<digit>> form in only the left hand side.
1477 Anywhere else it's $<I<digit>>.
1479 Occasionally, you can't use just a C</g> to get all the changes
1480 to occur that you might want. Here are two common cases:
1482 # put commas in the right places in an integer
1483 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
1485 # expand tabs to 8-column spacing
1486 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
1488 =item tr/SEARCHLIST/REPLACEMENTLIST/cds
1489 X<tr> X<y> X<transliterate> X</c> X</d> X</s>
1491 =item y/SEARCHLIST/REPLACEMENTLIST/cds
1493 Transliterates all occurrences of the characters found in the search list
1494 with the corresponding character in the replacement list. It returns
1495 the number of characters replaced or deleted. If no string is
1496 specified via the =~ or !~ operator, the $_ string is transliterated. (The
1497 string specified with =~ must be a scalar variable, an array element, a
1498 hash element, or an assignment to one of those, i.e., an lvalue.)
1500 A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
1501 does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
1502 For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
1503 SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
1504 its own pair of quotes, which may or may not be bracketing quotes,
1505 e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
1507 Note that C<tr> does B<not> do regular expression character classes
1508 such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
1509 the tr(1) utility. If you want to map strings between lower/upper
1510 cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
1511 using the C<s> operator if you need regular expressions.
1513 Note also that the whole range idea is rather unportable between
1514 character sets--and even within character sets they may cause results
1515 you probably didn't expect. A sound principle is to use only ranges
1516 that begin from and end at either alphabets of equal case (a-e, A-E),
1517 or digits (0-4). Anything else is unsafe. If in doubt, spell out the
1518 character sets in full.
1522 c Complement the SEARCHLIST.
1523 d Delete found but unreplaced characters.
1524 s Squash duplicate replaced characters.
1526 If the C</c> modifier is specified, the SEARCHLIST character set
1527 is complemented. If the C</d> modifier is specified, any characters
1528 specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
1529 (Note that this is slightly more flexible than the behavior of some
1530 B<tr> programs, which delete anything they find in the SEARCHLIST,
1531 period.) If the C</s> modifier is specified, sequences of characters
1532 that were transliterated to the same character are squashed down
1533 to a single instance of the character.
1535 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
1536 exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
1537 than the SEARCHLIST, the final character is replicated till it is long
1538 enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
1539 This latter is useful for counting characters in a class or for
1540 squashing character sequences in a class.
1544 $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
1546 $cnt = tr/*/*/; # count the stars in $_
1548 $cnt = $sky =~ tr/*/*/; # count the stars in $sky
1550 $cnt = tr/0-9//; # count the digits in $_
1552 tr/a-zA-Z//s; # bookkeeper -> bokeper
1554 ($HOST = $host) =~ tr/a-z/A-Z/;
1556 tr/a-zA-Z/ /cs; # change non-alphas to single space
1559 [\000-\177]; # delete 8th bit
1561 If multiple transliterations are given for a character, only the
1566 will transliterate any A to X.
1568 Because the transliteration table is built at compile time, neither
1569 the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
1570 interpolation. That means that if you want to use variables, you
1573 eval "tr/$oldlist/$newlist/";
1576 eval "tr/$oldlist/$newlist/, 1" or die $@;
1579 X<here-doc> X<heredoc> X<here-document> X<<< << >>>
1581 A line-oriented form of quoting is based on the shell "here-document"
1582 syntax. Following a C<< << >> you specify a string to terminate
1583 the quoted material, and all lines following the current line down to
1584 the terminating string are the value of the item. The terminating
1585 string may be either an identifier (a word), or some quoted text. If
1586 quoted, the type of quotes you use determines the treatment of the
1587 text, just as in regular quoting. An unquoted identifier works like
1588 double quotes. There must be no space between the C<< << >> and
1589 the identifier, unless the identifier is quoted. (If you put a space it
1590 will be treated as a null identifier, which is valid, and matches the first
1591 empty line.) The terminating string must appear by itself (unquoted and
1592 with no surrounding whitespace) on the terminating line.
1595 The price is $Price.
1598 print << "EOF"; # same as above
1599 The price is $Price.
1602 print << `EOC`; # execute commands
1607 print <<"foo", <<"bar"; # you can stack them
1613 myfunc(<< "THIS", 23, <<'THAT');
1620 Just don't forget that you have to put a semicolon on the end
1621 to finish the statement, as Perl doesn't know you're not going to
1629 If you want your here-docs to be indented with the
1630 rest of the code, you'll need to remove leading whitespace
1631 from each line manually:
1633 ($quote = <<'FINIS') =~ s/^\s+//gm;
1634 The Road goes ever on and on,
1635 down from the door where it began.
1638 If you use a here-doc within a delimited construct, such as in C<s///eg>,
1639 the quoted material must come on the lines following the final delimiter.
1654 If the terminating identifier is on the last line of the program, you
1655 must be sure there is a newline after it; otherwise, Perl will give the
1656 warning B<Can't find string terminator "END" anywhere before EOF...>.
1658 Additionally, the quoting rules for the identifier are not related to
1659 Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
1660 in place of C<''> and C<"">, and the only interpolation is for backslashing
1661 the quoting character:
1663 print << "abc\"def";
1667 Finally, quoted strings cannot span multiple lines. The general rule is
1668 that the identifier must be a string literal. Stick with that, and you
1673 =head2 Gory details of parsing quoted constructs
1674 X<quote, gory details>
1676 When presented with something that might have several different
1677 interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
1678 principle to pick the most probable interpretation. This strategy
1679 is so successful that Perl programmers often do not suspect the
1680 ambivalence of what they write. But from time to time, Perl's
1681 notions differ substantially from what the author honestly meant.
1683 This section hopes to clarify how Perl handles quoted constructs.
1684 Although the most common reason to learn this is to unravel labyrinthine
1685 regular expressions, because the initial steps of parsing are the
1686 same for all quoting operators, they are all discussed together.
1688 The most important Perl parsing rule is the first one discussed
1689 below: when processing a quoted construct, Perl first finds the end
1690 of that construct, then interprets its contents. If you understand
1691 this rule, you may skip the rest of this section on the first
1692 reading. The other rules are likely to contradict the user's
1693 expectations much less frequently than this first one.
1695 Some passes discussed below are performed concurrently, but because
1696 their results are the same, we consider them individually. For different
1697 quoting constructs, Perl performs different numbers of passes, from
1698 one to five, but these passes are always performed in the same order.
1702 =item Finding the end
1704 The first pass is finding the end of the quoted construct, whether
1705 it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
1706 construct, a C</> that terminates a C<qq//> construct, a C<]> which
1707 terminates C<qq[]> construct, or a C<< > >> which terminates a
1708 fileglob started with C<< < >>.
1710 When searching for single-character non-pairing delimiters, such
1711 as C</>, combinations of C<\\> and C<\/> are skipped. However,
1712 when searching for single-character pairing delimiter like C<[>,
1713 combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
1714 C<[>, C<]> are skipped as well. When searching for multicharacter
1715 delimiters, nothing is skipped.
1717 For constructs with three-part delimiters (C<s///>, C<y///>, and
1718 C<tr///>), the search is repeated once more.
1720 During this search no attention is paid to the semantics of the construct.
1723 "$hash{"$foo/$bar"}"
1728 bar # NOT a comment, this slash / terminated m//!
1731 do not form legal quoted expressions. The quoted part ends on the
1732 first C<"> and C</>, and the rest happens to be a syntax error.
1733 Because the slash that terminated C<m//> was followed by a C<SPACE>,
1734 the example above is not C<m//x>, but rather C<m//> with no C</x>
1735 modifier. So the embedded C<#> is interpreted as a literal C<#>.
1737 Also no attention is paid to C<\c\> during this search.
1738 Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
1739 and the following C</> is not recognized as a delimiter.
1740 Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
1742 =item Removal of backslashes before delimiters
1744 During the second pass, text between the starting and ending
1745 delimiters is copied to a safe location, and the C<\> is removed
1746 from combinations consisting of C<\> and delimiter--or delimiters,
1747 meaning both starting and ending delimiters will should these differ.
1748 This removal does not happen for multi-character delimiters.
1749 Note that the combination C<\\> is left intact, just as it was.
1751 Starting from this step no information about the delimiters is
1757 The next step is interpolation in the text obtained, which is now
1758 delimiter-independent. There are four different cases.
1762 =item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
1764 No interpolation is performed.
1768 The only interpolation is removal of C<\> from pairs C<\\>.
1770 =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
1772 C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
1773 converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
1774 is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
1775 The other combinations are replaced with appropriate expansions.
1777 Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
1778 is interpolated in the usual way. Something like C<"\Q\\E"> has
1779 no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
1780 result is the same as for C<"\\\\E">. As a general rule, backslashes
1781 between C<\Q> and C<\E> may lead to counterintuitive results. So,
1782 C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
1783 as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
1788 may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
1790 Interpolated scalars and arrays are converted internally to the C<join> and
1791 C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
1793 $foo . " XXX '" . (join $", @arr) . "'";
1795 All operations above are performed simultaneously, left to right.
1797 Because the result of C<"\Q STRING \E"> has all metacharacters
1798 quoted, there is no way to insert a literal C<$> or C<@> inside a
1799 C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
1800 C<"\\\$">; if not, it is interpreted as the start of an interpolated
1803 Note also that the interpolation code needs to make a decision on
1804 where the interpolated scalar ends. For instance, whether
1805 C<< "a $b -> {c}" >> really means:
1807 "a " . $b . " -> {c}";
1813 Most of the time, the longest possible text that does not include
1814 spaces between components and which contains matching braces or
1815 brackets. because the outcome may be determined by voting based
1816 on heuristic estimators, the result is not strictly predictable.
1817 Fortunately, it's usually correct for ambiguous cases.
1819 =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
1821 Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
1822 happens (almost) as with C<qq//> constructs, but the substitution
1823 of C<\> followed by RE-special chars (including C<\>) is not
1824 performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
1825 a C<#>-comment in a C<//x>-regular expression, no processing is
1826 performed whatsoever. This is the first step at which the presence
1827 of the C<//x> modifier is relevant.
1829 Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
1830 interpolated, and constructs C<$var[SOMETHING]> are voted (by several
1831 different estimators) to be either an array element or C<$var>
1832 followed by an RE alternative. This is where the notation
1833 C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
1834 array element C<-9>, not as a regular expression from the variable
1835 C<$arr> followed by a digit, which would be the interpretation of
1836 C</$arr[0-9]/>. Since voting among different estimators may occur,
1837 the result is not predictable.
1839 It is at this step that C<\1> is begrudgingly converted to C<$1> in
1840 the replacement text of C<s///> to correct the incorrigible
1841 I<sed> hackers who haven't picked up the saner idiom yet. A warning
1842 is emitted if the C<use warnings> pragma or the B<-w> command-line flag
1843 (that is, the C<$^W> variable) was set.
1845 The lack of processing of C<\\> creates specific restrictions on
1846 the post-processed text. If the delimiter is C</>, one cannot get
1847 the combination C<\/> into the result of this step. C</> will
1848 finish the regular expression, C<\/> will be stripped to C</> on
1849 the previous step, and C<\\/> will be left as is. Because C</> is
1850 equivalent to C<\/> inside a regular expression, this does not
1851 matter unless the delimiter happens to be character special to the
1852 RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
1853 alphanumeric char, as in:
1857 In the RE above, which is intentionally obfuscated for illustration, the
1858 delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
1859 RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
1860 reason you're encouraged to restrict your delimiters to non-alphanumeric,
1861 non-whitespace choices.
1865 This step is the last one for all constructs except regular expressions,
1866 which are processed further.
1868 =item Interpolation of regular expressions
1869 X<regexp, interpolation>
1871 Previous steps were performed during the compilation of Perl code,
1872 but this one happens at run time--although it may be optimized to
1873 be calculated at compile time if appropriate. After preprocessing
1874 described above, and possibly after evaluation if catenation,
1875 joining, casing translation, or metaquoting are involved, the
1876 resulting I<string> is passed to the RE engine for compilation.
1878 Whatever happens in the RE engine might be better discussed in L<perlre>,
1879 but for the sake of continuity, we shall do so here.
1881 This is another step where the presence of the C<//x> modifier is
1882 relevant. The RE engine scans the string from left to right and
1883 converts it to a finite automaton.
1885 Backslashed characters are either replaced with corresponding
1886 literal strings (as with C<\{>), or else they generate special nodes
1887 in the finite automaton (as with C<\b>). Characters special to the
1888 RE engine (such as C<|>) generate corresponding nodes or groups of
1889 nodes. C<(?#...)> comments are ignored. All the rest is either
1890 converted to literal strings to match, or else is ignored (as is
1891 whitespace and C<#>-style comments if C<//x> is present).
1893 Parsing of the bracketed character class construct, C<[...]>, is
1894 rather different than the rule used for the rest of the pattern.
1895 The terminator of this construct is found using the same rules as
1896 for finding the terminator of a C<{}>-delimited construct, the only
1897 exception being that C<]> immediately following C<[> is treated as
1898 though preceded by a backslash. Similarly, the terminator of
1899 C<(?{...})> is found using the same rules as for finding the
1900 terminator of a C<{}>-delimited construct.
1902 It is possible to inspect both the string given to RE engine and the
1903 resulting finite automaton. See the arguments C<debug>/C<debugcolor>
1904 in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
1905 switch documented in L<perlrun/"Command Switches">.
1907 =item Optimization of regular expressions
1908 X<regexp, optimization>
1910 This step is listed for completeness only. Since it does not change
1911 semantics, details of this step are not documented and are subject
1912 to change without notice. This step is performed over the finite
1913 automaton that was generated during the previous pass.
1915 It is at this stage that C<split()> silently optimizes C</^/> to
1920 =head2 I/O Operators
1921 X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
1924 There are several I/O operators you should know about.
1926 A string enclosed by backticks (grave accents) first undergoes
1927 double-quote interpolation. It is then interpreted as an external
1928 command, and the output of that command is the value of the
1929 backtick string, like in a shell. In scalar context, a single string
1930 consisting of all output is returned. In list context, a list of
1931 values is returned, one per line of output. (You can set C<$/> to use
1932 a different line terminator.) The command is executed each time the
1933 pseudo-literal is evaluated. The status value of the command is
1934 returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
1935 Unlike in B<csh>, no translation is done on the return data--newlines
1936 remain newlines. Unlike in any of the shells, single quotes do not
1937 hide variable names in the command from interpretation. To pass a
1938 literal dollar-sign through to the shell you need to hide it with a
1939 backslash. The generalized form of backticks is C<qx//>. (Because
1940 backticks always undergo shell expansion as well, see L<perlsec> for
1942 X<qx> X<`> X<``> X<backtick> X<glob>
1944 In scalar context, evaluating a filehandle in angle brackets yields
1945 the next line from that file (the newline, if any, included), or
1946 C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
1947 (sometimes known as file-slurp mode) and the file is empty, it
1948 returns C<''> the first time, followed by C<undef> subsequently.
1950 Ordinarily you must assign the returned value to a variable, but
1951 there is one situation where an automatic assignment happens. If
1952 and only if the input symbol is the only thing inside the conditional
1953 of a C<while> statement (even if disguised as a C<for(;;)> loop),
1954 the value is automatically assigned to the global variable $_,
1955 destroying whatever was there previously. (This may seem like an
1956 odd thing to you, but you'll use the construct in almost every Perl
1957 script you write.) The $_ variable is not implicitly localized.
1958 You'll have to put a C<local $_;> before the loop if you want that
1961 The following lines are equivalent:
1963 while (defined($_ = <STDIN>)) { print; }
1964 while ($_ = <STDIN>) { print; }
1965 while (<STDIN>) { print; }
1966 for (;<STDIN>;) { print; }
1967 print while defined($_ = <STDIN>);
1968 print while ($_ = <STDIN>);
1969 print while <STDIN>;
1971 This also behaves similarly, but avoids $_ :
1973 while (my $line = <STDIN>) { print $line }
1975 In these loop constructs, the assigned value (whether assignment
1976 is automatic or explicit) is then tested to see whether it is
1977 defined. The defined test avoids problems where line has a string
1978 value that would be treated as false by Perl, for example a "" or
1979 a "0" with no trailing newline. If you really mean for such values
1980 to terminate the loop, they should be tested for explicitly:
1982 while (($_ = <STDIN>) ne '0') { ... }
1983 while (<STDIN>) { last unless $_; ... }
1985 In other boolean contexts, C<< <I<filehandle>> >> without an
1986 explicit C<defined> test or comparison elicit a warning if the
1987 C<use warnings> pragma or the B<-w>
1988 command-line switch (the C<$^W> variable) is in effect.
1990 The filehandles STDIN, STDOUT, and STDERR are predefined. (The
1991 filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
1992 in packages, where they would be interpreted as local identifiers
1993 rather than global.) Additional filehandles may be created with
1994 the open() function, amongst others. See L<perlopentut> and
1995 L<perlfunc/open> for details on this.
1996 X<stdin> X<stdout> X<sterr>
1998 If a <FILEHANDLE> is used in a context that is looking for
1999 a list, a list comprising all input lines is returned, one line per
2000 list element. It's easy to grow to a rather large data space this
2001 way, so use with care.
2003 <FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
2004 See L<perlfunc/readline>.
2006 The null filehandle <> is special: it can be used to emulate the
2007 behavior of B<sed> and B<awk>. Input from <> comes either from
2008 standard input, or from each file listed on the command line. Here's
2009 how it works: the first time <> is evaluated, the @ARGV array is
2010 checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
2011 gives you standard input. The @ARGV array is then processed as a list
2012 of filenames. The loop
2015 ... # code for each line
2018 is equivalent to the following Perl-like pseudo code:
2020 unshift(@ARGV, '-') unless @ARGV;
2021 while ($ARGV = shift) {
2024 ... # code for each line
2028 except that it isn't so cumbersome to say, and will actually work.
2029 It really does shift the @ARGV array and put the current filename
2030 into the $ARGV variable. It also uses filehandle I<ARGV>
2031 internally--<> is just a synonym for <ARGV>, which
2032 is magical. (The pseudo code above doesn't work because it treats
2033 <ARGV> as non-magical.)
2035 You can modify @ARGV before the first <> as long as the array ends up
2036 containing the list of filenames you really want. Line numbers (C<$.>)
2037 continue as though the input were one big happy file. See the example
2038 in L<perlfunc/eof> for how to reset line numbers on each file.
2040 If you want to set @ARGV to your own list of files, go right ahead.
2041 This sets @ARGV to all plain text files if no @ARGV was given:
2043 @ARGV = grep { -f && -T } glob('*') unless @ARGV;
2045 You can even set them to pipe commands. For example, this automatically
2046 filters compressed arguments through B<gzip>:
2048 @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
2050 If you want to pass switches into your script, you can use one of the
2051 Getopts modules or put a loop on the front like this:
2053 while ($_ = $ARGV[0], /^-/) {
2056 if (/^-D(.*)/) { $debug = $1 }
2057 if (/^-v/) { $verbose++ }
2058 # ... # other switches
2062 # ... # code for each line
2065 The <> symbol will return C<undef> for end-of-file only once.
2066 If you call it again after this, it will assume you are processing another
2067 @ARGV list, and if you haven't set @ARGV, will read input from STDIN.
2069 If what the angle brackets contain is a simple scalar variable (e.g.,
2070 <$foo>), then that variable contains the name of the
2071 filehandle to input from, or its typeglob, or a reference to the
2077 If what's within the angle brackets is neither a filehandle nor a simple
2078 scalar variable containing a filehandle name, typeglob, or typeglob
2079 reference, it is interpreted as a filename pattern to be globbed, and
2080 either a list of filenames or the next filename in the list is returned,
2081 depending on context. This distinction is determined on syntactic
2082 grounds alone. That means C<< <$x> >> is always a readline() from
2083 an indirect handle, but C<< <$hash{key}> >> is always a glob().
2084 That's because $x is a simple scalar variable, but C<$hash{key}> is
2085 not--it's a hash element. Even C<< <$x > >> (note the extra space)
2086 is treated as C<glob("$x ")>, not C<readline($x)>.
2088 One level of double-quote interpretation is done first, but you can't
2089 say C<< <$foo> >> because that's an indirect filehandle as explained
2090 in the previous paragraph. (In older versions of Perl, programmers
2091 would insert curly brackets to force interpretation as a filename glob:
2092 C<< <${foo}> >>. These days, it's considered cleaner to call the
2093 internal function directly as C<glob($foo)>, which is probably the right
2094 way to have done it in the first place.) For example:
2100 is roughly equivalent to:
2102 open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
2108 except that the globbing is actually done internally using the standard
2109 C<File::Glob> extension. Of course, the shortest way to do the above is:
2113 A (file)glob evaluates its (embedded) argument only when it is
2114 starting a new list. All values must be read before it will start
2115 over. In list context, this isn't important because you automatically
2116 get them all anyway. However, in scalar context the operator returns
2117 the next value each time it's called, or C<undef> when the list has
2118 run out. As with filehandle reads, an automatic C<defined> is
2119 generated when the glob occurs in the test part of a C<while>,
2120 because legal glob returns (e.g. a file called F<0>) would otherwise
2121 terminate the loop. Again, C<undef> is returned only once. So if
2122 you're expecting a single value from a glob, it is much better to
2125 ($file) = <blurch*>;
2131 because the latter will alternate between returning a filename and
2134 If you're trying to do variable interpolation, it's definitely better
2135 to use the glob() function, because the older notation can cause people
2136 to become confused with the indirect filehandle notation.
2138 @files = glob("$dir/*.[ch]");
2139 @files = glob($files[$i]);
2141 =head2 Constant Folding
2142 X<constant folding> X<folding>
2144 Like C, Perl does a certain amount of expression evaluation at
2145 compile time whenever it determines that all arguments to an
2146 operator are static and have no side effects. In particular, string
2147 concatenation happens at compile time between literals that don't do
2148 variable substitution. Backslash interpolation also happens at
2149 compile time. You can say
2151 'Now is the time for all' . "\n" .
2152 'good men to come to.'
2154 and this all reduces to one string internally. Likewise, if
2157 foreach $file (@filenames) {
2158 if (-s $file > 5 + 100 * 2**16) { }
2161 the compiler will precompute the number which that expression
2162 represents so that the interpreter won't have to.
2167 Perl doesn't officially have a no-op operator, but the bare constants
2168 C<0> and C<1> are special-cased to not produce a warning in a void
2169 context, so you can for example safely do
2173 =head2 Bitwise String Operators
2174 X<operator, bitwise, string>
2176 Bitstrings of any size may be manipulated by the bitwise operators
2179 If the operands to a binary bitwise op are strings of different
2180 sizes, B<|> and B<^> ops act as though the shorter operand had
2181 additional zero bits on the right, while the B<&> op acts as though
2182 the longer operand were truncated to the length of the shorter.
2183 The granularity for such extension or truncation is one or more
2186 # ASCII-based examples
2187 print "j p \n" ^ " a h"; # prints "JAPH\n"
2188 print "JA" | " ph\n"; # prints "japh\n"
2189 print "japh\nJunk" & '_____'; # prints "JAPH\n";
2190 print 'p N$' ^ " E<H\n"; # prints "Perl\n";
2192 If you are intending to manipulate bitstrings, be certain that
2193 you're supplying bitstrings: If an operand is a number, that will imply
2194 a B<numeric> bitwise operation. You may explicitly show which type of
2195 operation you intend by using C<""> or C<0+>, as in the examples below.
2197 $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
2198 $foo = '150' | 105; # yields 255
2199 $foo = 150 | '105'; # yields 255
2200 $foo = '150' | '105'; # yields string '155' (under ASCII)
2202 $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
2203 $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
2205 See L<perlfunc/vec> for information on how to manipulate individual bits
2208 =head2 Integer Arithmetic
2211 By default, Perl assumes that it must do most of its arithmetic in
2212 floating point. But by saying
2216 you may tell the compiler that it's okay to use integer operations
2217 (if it feels like it) from here to the end of the enclosing BLOCK.
2218 An inner BLOCK may countermand this by saying
2222 which lasts until the end of that BLOCK. Note that this doesn't
2223 mean everything is only an integer, merely that Perl may use integer
2224 operations if it is so inclined. For example, even under C<use
2225 integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
2228 Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
2229 and ">>") always produce integral results. (But see also
2230 L<Bitwise String Operators>.) However, C<use integer> still has meaning for
2231 them. By default, their results are interpreted as unsigned integers, but
2232 if C<use integer> is in effect, their results are interpreted
2233 as signed integers. For example, C<~0> usually evaluates to a large
2234 integral value. However, C<use integer; ~0> is C<-1> on twos-complement
2237 =head2 Floating-point Arithmetic
2238 X<floating-point> X<floating point> X<float> X<real>
2240 While C<use integer> provides integer-only arithmetic, there is no
2241 analogous mechanism to provide automatic rounding or truncation to a
2242 certain number of decimal places. For rounding to a certain number
2243 of digits, sprintf() or printf() is usually the easiest route.
2246 Floating-point numbers are only approximations to what a mathematician
2247 would call real numbers. There are infinitely more reals than floats,
2248 so some corners must be cut. For example:
2250 printf "%.20g\n", 123456789123456789;
2251 # produces 123456789123456784
2253 Testing for exact equality of floating-point equality or inequality is
2254 not a good idea. Here's a (relatively expensive) work-around to compare
2255 whether two floating-point numbers are equal to a particular number of
2256 decimal places. See Knuth, volume II, for a more robust treatment of
2260 my ($X, $Y, $POINTS) = @_;
2262 $tX = sprintf("%.${POINTS}g", $X);
2263 $tY = sprintf("%.${POINTS}g", $Y);
2267 The POSIX module (part of the standard perl distribution) implements
2268 ceil(), floor(), and other mathematical and trigonometric functions.
2269 The Math::Complex module (part of the standard perl distribution)
2270 defines mathematical functions that work on both the reals and the
2271 imaginary numbers. Math::Complex not as efficient as POSIX, but
2272 POSIX can't work with complex numbers.
2274 Rounding in financial applications can have serious implications, and
2275 the rounding method used should be specified precisely. In these
2276 cases, it probably pays not to trust whichever system rounding is
2277 being used by Perl, but to instead implement the rounding function you
2280 =head2 Bigger Numbers
2281 X<number, arbitrary precision>
2283 The standard Math::BigInt and Math::BigFloat modules provide
2284 variable-precision arithmetic and overloaded operators, although
2285 they're currently pretty slow. At the cost of some space and
2286 considerable speed, they avoid the normal pitfalls associated with
2287 limited-precision representations.
2290 $x = Math::BigInt->new('123456789123456789');
2293 # prints +15241578780673678515622620750190521
2295 There are several modules that let you calculate with (bound only by
2296 memory and cpu-time) unlimited or fixed precision. There are also
2297 some non-standard modules that provide faster implementations via
2298 external C libraries.
2300 Here is a short, but incomplete summary:
2302 Math::Fraction big, unlimited fractions like 9973 / 12967
2303 Math::String treat string sequences like numbers
2304 Math::FixedPrecision calculate with a fixed precision
2305 Math::Currency for currency calculations
2306 Bit::Vector manipulate bit vectors fast (uses C)
2307 Math::BigIntFast Bit::Vector wrapper for big numbers
2308 Math::Pari provides access to the Pari C library
2309 Math::BigInteger uses an external C library
2310 Math::Cephes uses external Cephes C library (no big numbers)
2311 Math::Cephes::Fraction fractions via the Cephes library
2312 Math::GMP another one using an external C library