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*pattern.txt*   For Vim version 7.2.  Last change: 2008 Sep 05


                  VIM REFERENCE MANUAL    by Bram Moolenaar


Patterns and search commands                            *pattern-searches*

The very basics can be found in section |03.9| of the user manual.  A few more
explanations are in chapter 27 |usr_27.txt|.

1. Search commands              |search-commands|
2. The definition of a pattern  |search-pattern|
3. Magic                        |/magic|
4. Overview of pattern items    |pattern-overview|
5. Multi items                  |pattern-multi-items|
6. Ordinary atoms               |pattern-atoms|
7. Ignoring case in a pattern   |/ignorecase|
8. Composing characters         |patterns-composing|
9. Compare with Perl patterns   |perl-patterns|
10. Highlighting matches        |match-highlight|

==============================================================================
1. Search commands                              *search-commands* *E486*

                                                        */*
/{pattern}[/]<CR>       Search forward for the [count]'th occurrence of
                        {pattern} |exclusive|.

/{pattern}/{offset}<CR> Search forward for the [count]'th occurrence of
                        {pattern} and go |{offset}| lines up or down.
                        |linewise|.

                                                        */<CR>*
/<CR>                   Search forward for the [count]'th occurrence of the
                        latest used pattern |last-pattern| with latest used
                        |{offset}|.

//{offset}<CR>          Search forward for the [count]'th occurrence of the
                        latest used pattern |last-pattern| with new
                        |{offset}|.  If {offset} is empty no offset is used.

                                                        *?*
?{pattern}[?]<CR>       Search backward for the [count]'th previous
                        occurrence of {pattern} |exclusive|.

?{pattern}?{offset}<CR> Search backward for the [count]'th previous
                        occurrence of {pattern} and go |{offset}| lines up or
                        down |linewise|.

                                                        *?<CR>*
?<CR>                   Search backward for the [count]'th occurrence of the
                        latest used pattern |last-pattern| with latest used
                        |{offset}|.

??{offset}<CR>          Search backward for the [count]'th occurrence of the
                        latest used pattern |last-pattern| with new
                        |{offset}|.  If {offset} is empty no offset is used.

                                                        *n*
n                       Repeat the latest "/" or "?" [count] times.
                        |last-pattern| {Vi: no count}

                                                        *N*
N                       Repeat the latest "/" or "?" [count] times in
                        opposite direction. |last-pattern| {Vi: no count}

                                                        *star* *E348* *E349*
*                       Search forward for the [count]'th occurrence of the
                        word nearest to the cursor.  The word used for the
                        search is the first of:
                                1. the keyword under the cursor |'iskeyword'|
                                2. the first keyword after the cursor, in the
                                   current line
                                3. the non-blank word under the cursor
                                4. the first non-blank word after the cursor,
                                   in the current line
                        Only whole keywords are searched for, like with the
                        command "/\<keyword\>".  |exclusive|  {not in Vi}
                        'ignorecase' is used, 'smartcase' is not.

                                                        *#*
#                       Same as "*", but search backward.  The pound sign
                        (character 163) also works.  If the "#" key works as
                        backspace, try using "stty erase <BS>" before starting
                        Vim (<BS> is CTRL-H or a real backspace).  {not in Vi}

                                                        *gstar*
g*                      Like "*", but don't put "\<" and "\>" around the word.
                        This makes the search also find matches that are not a
                        whole word.  {not in Vi}

                                                        *g#*
g#                      Like "#", but don't put "\<" and "\>" around the word.
                        This makes the search also find matches that are not a
                        whole word.  {not in Vi}

                                                        *gd*
gd                      Goto local Declaration.  When the cursor is on a local
                        variable, this command will jump to its declaration.
                        First Vim searches for the start of the current
                        function, just like "[[".  If it is not found the
                        search stops in line 1.  If it is found, Vim goes back
                        until a blank line is found.  From this position Vim
                        searches for the keyword under the cursor, like with
                        "*", but lines that look like a comment are ignored
                        (see 'comments' option).
                        Note that this is not guaranteed to work, Vim does not
                        really check the syntax, it only searches for a match
                        with the keyword.  If included files also need to be
                        searched use the commands listed in |include-search|.
                        After this command |n| searches forward for the next
                        match (not backward).
                        {not in Vi}

                                                        *gD*
gD                      Goto global Declaration.  When the cursor is on a
                        global variable that is defined in the file, this
                        command will jump to its declaration.  This works just
                        like "gd", except that the search for the keyword
                        always starts in line 1.  {not in Vi}

                                                        *1gd*
1gd                     Like "gd", but ignore matches inside a {} block that
                        ends before the cursor position. {not in Vi}

                                                        *1gD*
1gD                     Like "gD", but ignore matches inside a {} block that
                        ends before the cursor position. {not in Vi}

                                                        *CTRL-C*
CTRL-C                  Interrupt current (search) command.  Use CTRL-Break on
                        MS-DOS |dos-CTRL-Break|.
                        In Normal mode, any pending command is aborted.

                                                        *:noh* *:nohlsearch*
:noh[lsearch]           Stop the highlighting for the 'hlsearch' option.  It
                        is automatically turned back on when using a search
                        command, or setting the 'hlsearch' option.
                        This command doesn't work in an autocommand, because
                        the highlighting state is saved and restored when
                        executing autocommands |autocmd-searchpat|.
                        Same thing for when invoking a user function.

While typing the search pattern the current match will be shown if the
'incsearch' option is on.  Remember that you still have to finish the search
command with <CR> to actually position the cursor at the displayed match.  Or
use <Esc> to abandon the search.

All matches for the last used search pattern will be highlighted if you set
the 'hlsearch' option.  This can be suspended with the |:nohlsearch| command.

                                        *search-offset* *{offset}*
These commands search for the specified pattern.  With "/" and "?" an
additional offset may be given.  There are two types of offsets: line offsets
and character offsets.  {the character offsets are not in Vi}

The offset gives the cursor position relative to the found match:
    [num]       [num] lines downwards, in column 1
    +[num]      [num] lines downwards, in column 1
    -[num]      [num] lines upwards, in column 1
    e[+num]     [num] characters to the right of the end of the match
    e[-num]     [num] characters to the left of the end of the match
    s[+num]     [num] characters to the right of the start of the match
    s[-num]     [num] characters to the left of the start of the match
    b[+num]     [num] identical to s[+num] above (mnemonic: begin)
    b[-num]     [num] identical to s[-num] above (mnemonic: begin)
    ;{pattern}  perform another search, see |//;|

If a '-' or '+' is given but [num] is omitted, a count of one will be used.
When including an offset with 'e', the search becomes inclusive (the
character the cursor lands on is included in operations).

Examples:

pattern                 cursor position ~
/test/+1                one line below "test", in column 1
/test/e                 on the last t of "test"
/test/s+2               on the 's' of "test"
/test/b-3               three characters before "test"

If one of these commands is used after an operator, the characters between
the cursor position before and after the search is affected.  However, if a
line offset is given, the whole lines between the two cursor positions are
affected.

An example of how to search for matches with a pattern and change the match
with another word: >
        /foo<CR>        find "foo"
        c//e            change until end of match
        bar<Esc>        type replacement
        //<CR>          go to start of next match
        c//e            change until end of match
        beep<Esc>       type another replacement
                        etc.
<
                                                        *//;* *E386*
A very special offset is ';' followed by another search command.  For example: >

   /test 1/;/test
   /test.*/+1;?ing?

The first one first finds the next occurrence of "test 1", and then the first
occurrence of "test" after that.

This is like executing two search commands after each other, except that:
- It can be used as a single motion command after an operator.
- The direction for a following "n" or "N" command comes from the first
  search command.
- When an error occurs the cursor is not moved at all.

                                                        *last-pattern*
The last used pattern and offset are remembered.  They can be used to repeat
the search, possibly in another direction or with another count.  Note that
two patterns are remembered: One for 'normal' search commands and one for the
substitute command ":s".  Each time an empty pattern is given, the previously
used pattern is used.

The 'magic' option sticks with the last used pattern.  If you change 'magic',
this will not change how the last used pattern will be interpreted.
The 'ignorecase' option does not do this.  When 'ignorecase' is changed, it
will result in the pattern to match other text.

All matches for the last used search pattern will be highlighted if you set
the 'hlsearch' option.

To clear the last used search pattern: >
        :let @/ = ""
This will not set the pattern to an empty string, because that would match
everywhere.  The pattern is really cleared, like when starting Vim.

The search usually skips matches that don't move the cursor.  Whether the next
match is found at the next character or after the skipped match depends on the
'c' flag in 'cpoptions'.  See |cpo-c|.
           with 'c' flag:   "/..." advances 1 to 3 characters
        without 'c' flag:   "/..." advances 1 character
The unpredictability with the 'c' flag is caused by starting the search in the
first column, skipping matches until one is found past the cursor position.

When searching backwards, searching starts at the start of the line, using the
'c' flag in 'cpoptions' as described above.  Then the last match before the
cursor position is used.

In Vi the ":tag" command sets the last search pattern when the tag is searched
for.  In Vim this is not done, the previous search pattern is still remembered,
unless the 't' flag is present in 'cpoptions'.  The search pattern is always
put in the search history.

If the 'wrapscan' option is on (which is the default), searches wrap around
the end of the buffer.  If 'wrapscan' is not set, the backward search stops
at the beginning and the forward search stops at the end of the buffer.  If
'wrapscan' is set and the pattern was not found the error message "pattern
not found" is given, and the cursor will not be moved.  If 'wrapscan' is not
set the message becomes "search hit BOTTOM without match" when searching
forward, or "search hit TOP without match" when searching backward.  If
wrapscan is set and the search wraps around the end of the file the message
"search hit TOP, continuing at BOTTOM" or "search hit BOTTOM, continuing at
TOP" is given when searching backwards or forwards respectively.  This can be
switched off by setting the 's' flag in the 'shortmess' option.  The highlight
method 'w' is used for this message (default: standout).

                                                        *search-range*
You can limit the search command "/" to a certain range of lines by including
\%>l items.  For example, to match the word "limit" below line 199 and above
line 300: >
        /\%>199l\%<300llimit
Also see |/\%>l|.

Another way is to use the ":substitute" command with the 'c' flag.  Example: >
   :.,300s/Pattern//gc
This command will search from the cursor position until line 300 for
"Pattern".  At the match, you will be asked to type a character.  Type 'q' to
stop at this match, type 'n' to find the next match.

The "*", "#", "g*" and "g#" commands look for a word near the cursor in this
order, the first one that is found is used:
- The keyword currently under the cursor.
- The first keyword to the right of the cursor, in the same line.
- The WORD currently under the cursor.
- The first WORD to the right of the cursor, in the same line.
The keyword may only contain letters and characters in 'iskeyword'.
The WORD may contain any non-blanks (<Tab>s and/or <Space>s).
Note that if you type with ten fingers, the characters are easy to remember:
the "#" is under your left hand middle finger (search to the left and up) and
the "*" is under your right hand middle finger (search to the right and down).
(this depends on your keyboard layout though).

==============================================================================
2. The definition of a pattern          *search-pattern* *pattern* *[pattern]*
                                        *regular-expression* *regexp* *Pattern*
                                        *E76* *E383* *E476*

For starters, read chapter 27 of the user manual |usr_27.txt|.

                                                */bar* */\bar* */pattern*
1. A pattern is one or more branches, separated by "\|".  It matches anything
   that matches one of the branches.  Example: "foo\|beep" matches "foo" and
   matches "beep".  If more than one branch matches, the first one is used.

   pattern ::=      branch
                or  branch \| branch
                or  branch \| branch \| branch
                etc.

                                                */branch* */\&*
2. A branch is one or more concats, separated by "\&".  It matches the last
   concat, but only if all the preceding concats also match at the same
   position.  Examples:
        "foobeep\&..." matches "foo" in "foobeep".
        ".*Peter\&.*Bob" matches in a line containing both "Peter" and "Bob"

   branch ::=       concat
                or  concat \& concat
                or  concat \& concat \& concat
                etc.

                                                */concat*
3. A concat is one or more pieces, concatenated.  It matches a match for the
   first piece, followed by a match for the second piece, etc.  Example:
   "f[0-9]b", first matches "f", then a digit and then "b".

   concat  ::=      piece
                or  piece piece
                or  piece piece piece
                etc.

                                                */piece*
4. A piece is an atom, possibly followed by a multi, an indication of how many
   times the atom can be matched.  Example: "a*" matches any sequence of "a"
   characters: "", "a", "aa", etc.  See |/multi|.

   piece   ::=      atom
                or  atom  multi

                                                */atom*
5. An atom can be one of a long list of items.  Many atoms match one character
   in the text.  It is often an ordinary character or a character class.
   Braces can be used to make a pattern into an atom.  The "\z(\)" construct
   is only for syntax highlighting.

   atom    ::=      ordinary-atom               |/ordinary-atom|
                or  \( pattern \)               |/\(|
                or  \%( pattern \)              |/\%(|
                or  \z( pattern \)              |/\z(|


==============================================================================
3. Magic                                                        */magic*

Some characters in the pattern are taken literally.  They match with the same
character in the text.  When preceded with a backslash however, these
characters get a special meaning.

Other characters have a special meaning without a backslash.  They need to be
preceded with a backslash to match literally.

If a character is taken literally or not depends on the 'magic' option and the
items mentioned next.
                                                        */\m* */\M*
Use of "\m" makes the pattern after it be interpreted as if 'magic' is set,
ignoring the actual value of the 'magic' option.
Use of "\M" makes the pattern after it be interpreted as if 'nomagic' is used.
                                                        */\v* */\V*
Use of "\v" means that in the pattern after it all ASCII characters except
'0'-'9', 'a'-'z', 'A'-'Z' and '_' have a special meaning.  "very magic"

Use of "\V" means that in the pattern after it only the backslash has a
special meaning.  "very nomagic"

Examples:
after:    \v       \m       \M       \V         matches ~
                'magic' 'nomagic'
          $        $        $        \$         matches end-of-line
          .        .        \.       \.         matches any character
          *        *        \*       \*         any number of the previous atom
          ()       \(\)     \(\)     \(\)       grouping into an atom
          |        \|       \|       \|         separating alternatives
          \a       \a       \a       \a         alphabetic character
          \\       \\       \\       \\         literal backslash
          \.       \.       .        .          literal dot
          \{       {        {        {          literal '{'
          a        a        a        a          literal 'a'

{only Vim supports \m, \M, \v and \V}

It is recommended to always keep the 'magic' option at the default setting,
which is 'magic'.  This avoids portability problems.  To make a pattern immune
to the 'magic' option being set or not, put "\m" or "\M" at the start of the
pattern.

==============================================================================
4. Overview of pattern items                            *pattern-overview*

Overview of multi items.                                */multi* *E61* *E62*
More explanation and examples below, follow the links.                  *E64*

          multi ~
     'magic' 'nomagic'  matches of the preceding atom ~
|/star| *       \*      0 or more       as many as possible
|/\+|   \+      \+      1 or more       as many as possible (*)
|/\=|   \=      \=      0 or 1          as many as possible (*)
|/\?|   \?      \?      0 or 1          as many as possible (*)

|/\{|   \{n,m}  \{n,m}  n to m          as many as possible (*)
        \{n}    \{n}    n               exactly (*)
        \{n,}   \{n,}   at least n      as many as possible (*)
        \{,m}   \{,m}   0 to m          as many as possible (*)
        \{}     \{}     0 or more       as many as possible (same as *) (*)

|/\{-|  \{-n,m} \{-n,m} n to m          as few as possible (*)
        \{-n}   \{-n}   n               exactly (*)
        \{-n,}  \{-n,}  at least n      as few as possible (*)
        \{-,m}  \{-,m}  0 to m          as few as possible (*)
        \{-}    \{-}    0 or more       as few as possible (*)

                                                        *E59*
|/\@>|  \@>     \@>     1, like matching a whole pattern (*)
|/\@=|  \@=     \@=     nothing, requires a match |/zero-width| (*)
|/\@!|  \@!     \@!     nothing, requires NO match |/zero-width| (*)
|/\@<=| \@<=    \@<=    nothing, requires a match behind |/zero-width| (*)
|/\@<!| \@<!    \@<!    nothing, requires NO match behind |/zero-width| (*)

(*) {not in Vi}


Overview of ordinary atoms.                             */ordinary-atom*
More explanation and examples below, follow the links.

      ordinary atom ~
      magic   nomagic   matches ~
|/^|    ^       ^       start-of-line (at start of pattern) |/zero-width|
|/\^|   \^      \^      literal '^'
|/\_^|  \_^     \_^     start-of-line (used anywhere) |/zero-width|
|/$|    $       $       end-of-line (at end of pattern) |/zero-width|
|/\$|   \$      \$      literal '$'
|/\_$|  \_$     \_$     end-of-line (used anywhere) |/zero-width|
|/.|    .       \.      any single character (not an end-of-line)
|/\_.|  \_.     \_.     any single character or end-of-line
|/\<|   \<      \<      beginning of a word |/zero-width|
|/\>|   \>      \>      end of a word |/zero-width|
|/\zs|  \zs     \zs     anything, sets start of match
|/\ze|  \ze     \ze     anything, sets end of match
|/\%^|  \%^     \%^     beginning of file |/zero-width|         *E71*
|/\%$|  \%$     \%$     end of file |/zero-width|
|/\%V|  \%V     \%V     inside Visual area |/zero-width|
|/\%#|  \%#     \%#     cursor position |/zero-width|
|/\%'m| \%'m    \%'m    mark m position |/zero-width|
|/\%l|  \%23l   \%23l   in line 23 |/zero-width|
|/\%c|  \%23c   \%23c   in column 23 |/zero-width|
|/\%v|  \%23v   \%23v   in virtual column 23 |/zero-width|

Character classes {not in Vi}:                          */character-classes*
|/\i|   \i      \i      identifier character (see 'isident' option)
|/\I|   \I      \I      like "\i", but excluding digits
|/\k|   \k      \k      keyword character (see 'iskeyword' option)
|/\K|   \K      \K      like "\k", but excluding digits
|/\f|   \f      \f      file name character (see 'isfname' option)
|/\F|   \F      \F      like "\f", but excluding digits
|/\p|   \p      \p      printable character (see 'isprint' option)
|/\P|   \P      \P      like "\p", but excluding digits
|/\s|   \s      \s      whitespace character: <Space> and <Tab>
|/\S|   \S      \S      non-whitespace character; opposite of \s
|/\d|   \d      \d      digit:                          [0-9]
|/\D|   \D      \D      non-digit:                      [^0-9]
|/\x|   \x      \x      hex digit:                      [0-9A-Fa-f]
|/\X|   \X      \X      non-hex digit:                  [^0-9A-Fa-f]
|/\o|   \o      \o      octal digit:                    [0-7]
|/\O|   \O      \O      non-octal digit:                [^0-7]
|/\w|   \w      \w      word character:                 [0-9A-Za-z_]
|/\W|   \W      \W      non-word character:             [^0-9A-Za-z_]
|/\h|   \h      \h      head of word character:         [A-Za-z_]
|/\H|   \H      \H      non-head of word character:     [^A-Za-z_]
|/\a|   \a      \a      alphabetic character:           [A-Za-z]
|/\A|   \A      \A      non-alphabetic character:       [^A-Za-z]
|/\l|   \l      \l      lowercase character:            [a-z]
|/\L|   \L      \L      non-lowercase character:        [^a-z]
|/\u|   \u      \u      uppercase character:            [A-Z]
|/\U|   \U      \U      non-uppercase character         [^A-Z]
|/\_|   \_x     \_x     where x is any of the characters above: character
                        class with end-of-line included
(end of character classes)

|/\e|   \e      \e      <Esc>
|/\t|   \t      \t      <Tab>
|/\r|   \r      \r      <CR>
|/\b|   \b      \b      <BS>
|/\n|   \n      \n      end-of-line
|/~|    ~       \~      last given substitute string
|/\1|   \1      \1      same string as matched by first \(\) {not in Vi}
|/\2|   \2      \2      Like "\1", but uses second \(\)
           ...
|/\9|   \9      \9      Like "\1", but uses ninth \(\)
                                                                *E68*
|/\z1|  \z1     \z1     only for syntax highlighting, see |:syn-ext-match|
           ...
|/\z1|  \z9     \z9     only for syntax highlighting, see |:syn-ext-match|

        x       x       a character with no special meaning matches itself

|/[]|   []      \[]     any character specified inside the []
|/\%[]| \%[]    \%[]    a sequence of optionally matched atoms

|/\c|   \c      \c      ignore case, do not use the 'ignorecase' option
|/\C|   \C      \C      match case, do not use the 'ignorecase' option
|/\m|   \m      \m      'magic' on for the following chars in the pattern
|/\M|   \M      \M      'magic' off for the following chars in the pattern
|/\v|   \v      \v      the following chars in the pattern are "very magic"
|/\V|   \V      \V      the following chars in the pattern are "very nomagic"
|/\Z|   \Z      \Z      ignore differences in Unicode "combining characters".
                        Useful when searching voweled Hebrew or Arabic text.

|/\%d|  \%d     \%d     match specified decimal character (eg \%d123)
|/\%x|  \%x     \%x     match specified hex character (eg \%x2a)
|/\%o|  \%o     \%o     match specified octal character (eg \%o040)
|/\%u|  \%u     \%u     match specified multibyte character (eg \%u20ac)
|/\%U|  \%U     \%U     match specified large multibyte character (eg
                        \%U12345678)

Example                 matches ~
\<\I\i*         or
\<\h\w*
\<[a-zA-Z_][a-zA-Z0-9_]*
                        An identifier (e.g., in a C program).

\(\.$\|\. \)            A period followed by <EOL> or a space.

[.!?][])"']*\($\|[ ]\)  A search pattern that finds the end of a sentence,
                        with almost the same definition as the ")" command.

cat\Z                   Both "cat" and "càt" ("a" followed by 0x0300)
                        Does not match "càt" (character 0x00e0), even
                        though it may look the same.


==============================================================================
5. Multi items                                          *pattern-multi-items*

An atom can be followed by an indication of how many times the atom can be
matched and in what way.  This is called a multi.  See |/multi| for an
overview.

                                                */star* */\star* *E56*
*       (use \* when 'magic' is not set)
        Matches 0 or more of the preceding atom, as many as possible.
        Example  'nomagic'      matches ~
        a*         a\*          "", "a", "aa", "aaa", etc.
        .*         \.\*         anything, also an empty string, no end-of-line
        \_.*       \_.\*        everything up to the end of the buffer
        \_.*END    \_.\*END     everything up to and including the last "END"
                                in the buffer

        Exception: When "*" is used at the start of the pattern or just after
        "^" it matches the star character.

        Be aware that repeating "\_." can match a lot of text and take a long
        time.  For example, "\_.*END" matches all text from the current
        position to the last occurrence of "END" in the file.  Since the "*"
        will match as many as possible, this first skips over all lines until
        the end of the file and then tries matching "END", backing up one
        character at a time.

                                                        */\+* *E57*
\+      Matches 1 or more of the preceding atom, as many as possible. {not in
        Vi}
        Example         matches ~
        ^.\+$           any non-empty line
        \s\+            white space of at least one character

                                                        */\=*
\=      Matches 0 or 1 of the preceding atom, as many as possible. {not in Vi}
        Example         matches ~
        foo\=           "fo" and "foo"

                                                        */\?*
\?      Just like \=.  Cannot be used when searching backwards with the "?"
        command. {not in Vi}

                                                */\{* *E58* *E60* *E554*
\{n,m}  Matches n to m of the preceding atom, as many as possible
\{n}    Matches n of the preceding atom
\{n,}   Matches at least n of the preceding atom, as many as possible
\{,m}   Matches 0 to m of the preceding atom, as many as possible
\{}     Matches 0 or more of the preceding atom, as many as possible (like *)
                                                        */\{-*
\{-n,m} matches n to m of the preceding atom, as few as possible
\{-n}   matches n of the preceding atom
\{-n,}  matches at least n of the preceding atom, as few as possible
\{-,m}  matches 0 to m of the preceding atom, as few as possible
\{-}    matches 0 or more of the preceding atom, as few as possible
        {Vi does not have any of these}

        n and m are positive decimal numbers or zero
                                                                *non-greedy*
        If a "-" appears immediately after the "{", then a shortest match
        first algorithm is used (see example below).  In particular, "\{-}" is
        the same as "*" but uses the shortest match first algorithm.  BUT: A
        match that starts earlier is preferred over a shorter match: "a\{-}b"
        matches "aaab" in "xaaab".

        Example                 matches ~
        ab\{2,3}c               "abbc" or "abbbc"
        a\{5}                   "aaaaa"
        ab\{2,}c                "abbc", "abbbc", "abbbbc", etc.
        ab\{,3}c                "ac", "abc", "abbc" or "abbbc"
        a[bc]\{3}d              "abbbd", "abbcd", "acbcd", "acccd", etc.
        a\(bc\)\{1,2}d          "abcd" or "abcbcd"
        a[bc]\{-}[cd]           "abc" in "abcd"
        a[bc]*[cd]              "abcd" in "abcd"

        The } may optionally be preceded with a backslash: \{n,m\}.

                                                        */\@=*
\@=     Matches the preceding atom with zero width. {not in Vi}
        Like "(?=pattern)" in Perl.
        Example                 matches ~
        foo\(bar\)\@=           "foo" in "foobar"
        foo\(bar\)\@=foo        nothing
                                                        */zero-width*
        When using "\@=" (or "^", "$", "\<", "\>") no characters are included
        in the match.  These items are only used to check if a match can be
        made.  This can be tricky, because a match with following items will
        be done in the same position.  The last example above will not match
        "foobarfoo", because it tries match "foo" in the same position where
        "bar" matched.

        Note that using "\&" works the same as using "\@=": "foo\&.." is the
        same as "\(foo\)\@=..".  But using "\&" is easier, you don't need the
        braces.


                                                        */\@!*
\@!     Matches with zero width if the preceding atom does NOT match at the
        current position. |/zero-width| {not in Vi}
        Like '(?!pattern)" in Perl.
        Example                 matches ~
        foo\(bar\)\@!           any "foo" not followed by "bar"
        a.\{-}p\@!              "a", "ap", "app", etc. not followed by a "p"
        if \(\(then\)\@!.\)*$   "if " not followed by "then"

        Using "\@!" is tricky, because there are many places where a pattern
        does not match.  "a.*p\@!" will match from an "a" to the end of the
        line, because ".*" can match all characters in the line and the "p"
        doesn't match at the end of the line.  "a.\{-}p\@!" will match any
        "a", "ap", "aap", etc. that isn't followed by a "p", because the "."
        can match a "p" and "p\@!" doesn't match after that.

        You can't use "\@!" to look for a non-match before the matching
        position: "\(foo\)\@!bar" will match "bar" in "foobar", because at the
        position where "bar" matches, "foo" does not match.  To avoid matching
        "foobar" you could use "\(foo\)\@!...bar", but that doesn't match a
        bar at the start of a line.  Use "\(foo\)\@<!bar".

                                                        */\@<=*
\@<=    Matches with zero width if the preceding atom matches just before what
        follows. |/zero-width| {not in Vi}
        Like '(?<=pattern)" in Perl, but Vim allows non-fixed-width patterns.
        Example                 matches ~
        \(an\_s\+\)\@<=file     "file" after "an" and white space or an
                                end-of-line
        For speed it's often much better to avoid this multi.  Try using "\zs"
        instead |/\zs|.  To match the same as the above example:
                an\_s\+\zsfile

        "\@<=" and "\@<!" check for matches just before what follows.
        Theoretically these matches could start anywhere before this position.
        But to limit the time needed, only the line where what follows matches
        is searched, and one line before that (if there is one).  This should
        be sufficient to match most things and not be too slow.
        The part of the pattern after "\@<=" and "\@<!" are checked for a
        match first, thus things like "\1" don't work to reference \(\) inside
        the preceding atom.  It does work the other way around:
        Example                 matches ~
        \1\@<=,\([a-z]\+\)      ",abc" in "abc,abc"

                                                        */\@<!*
\@<!    Matches with zero width if the preceding atom does NOT match just
        before what follows.  Thus this matches if there is no position in the
        current or previous line where the atom matches such that it ends just
        before what follows.  |/zero-width| {not in Vi}
        Like '(?<!pattern)" in Perl, but Vim allows non-fixed-width patterns.
        The match with the preceding atom is made to end just before the match
        with what follows, thus an atom that ends in ".*" will work.
        Warning: This can be slow (because many positions need to be checked
        for a match).
        Example                 matches ~
        \(foo\)\@<!bar          any "bar" that's not in "foobar"
        \(\/\/.*\)\@<!in        "in" which is not after "//"

                                                        */\@>*
\@>     Matches the preceding atom like matching a whole pattern. {not in Vi}
        Like "(?>pattern)" in Perl.
        Example         matches ~
        \(a*\)\@>a      nothing (the "a*" takes all the "a"'s, there can't be
                        another one following)

        This matches the preceding atom as if it was a pattern by itself.  If
        it doesn't match, there is no retry with shorter sub-matches or
        anything.  Observe this difference: "a*b" and "a*ab" both match
        "aaab", but in the second case the "a*" matches only the first two
        "a"s.  "\(a*\)\@>ab" will not match "aaab", because the "a*" matches
        the "aaa" (as many "a"s as possible), thus the "ab" can't match.


==============================================================================
6.  Ordinary atoms                                      *pattern-atoms*

An ordinary atom can be:

                                                        */^*
^       At beginning of pattern or after "\|", "\(", "\%(" or "\n": matches
        start-of-line; at other positions, matches literal '^'. |/zero-width|
        Example         matches ~
        ^beep(          the start of the C function "beep" (probably).

                                                        */\^*
\^      Matches literal '^'.  Can be used at any position in the pattern.

                                                        */\_^*
\_^     Matches start-of-line. |/zero-width|  Can be used at any position in
        the pattern.
        Example         matches ~
        \_s*\_^foo      white space and blank lines and then "foo" at
                        start-of-line

                                                        */$*
$       At end of pattern or in front of "\|", "\)" or "\n" ('magic' on):
        matches end-of-line <EOL>; at other positions, matches literal '$'.
        |/zero-width|

                                                        */\$*
\$      Matches literal '$'.  Can be used at any position in the pattern.

                                                        */\_$*
\_$     Matches end-of-line. |/zero-width|  Can be used at any position in the
        pattern.  Note that "a\_$b" never matches, since "b" cannot match an
        end-of-line.  Use "a\nb" instead |/\n|.
        Example         matches ~
        foo\_$\_s*      "foo" at end-of-line and following white space and
                        blank lines

.       (with 'nomagic': \.)                            */.* */\.*
        Matches any single character, but not an end-of-line.

                                                        */\_.*
\_.     Matches any single character or end-of-line.
        Careful: "\_.*" matches all text to the end of the buffer!

                                                        */\<*
\<      Matches the beginning of a word: The next char is the first char of a
        word.  The 'iskeyword' option specifies what is a word character.
        |/zero-width|

                                                        */\>*
\>      Matches the end of a word: The previous char is the last char of a
        word.  The 'iskeyword' option specifies what is a word character.
        |/zero-width|

                                                        */\zs*
\zs     Matches at any position, and sets the start of the match there: The
        next char is the first char of the whole match. |/zero-width|
        Example: >
                /^\s*\zsif
<       matches an "if" at the start of a line, ignoring white space.
        Can be used multiple times, the last one encountered in a matching
        branch is used.  Example: >
                /\(.\{-}\zsFab\)\{3}
<       Finds the third occurrence of "Fab".
        {not in Vi} {not available when compiled without the +syntax feature}
                                                        */\ze*
\ze     Matches at any position, and sets the end of the match there: The
        previous char is the last char of the whole match. |/zero-width|
        Can be used multiple times, the last one encountered in a matching
        branch is used.
        Example: "end\ze\(if\|for\)" matches the "end" in "endif" and
        "endfor".
        {not in Vi} {not available when compiled without the +syntax feature}

                                                */\%^* *start-of-file*
\%^     Matches start of the file.  When matching with a string, matches the
        start of the string. {not in Vi}
        For example, to find the first "VIM" in a file: >
                /\%^\_.\{-}\zsVIM
<
                                                */\%$* *end-of-file*
\%$     Matches end of the file.  When matching with a string, matches the
        end of the string. {not in Vi}
        Note that this does NOT find the last "VIM" in a file: >
                /VIM\_.\{-}\%$
<       It will find the next VIM, because the part after it will always
        match.  This one will find the last "VIM" in the file: >
                /VIM\ze\(\(VIM\)\@!\_.\)*\%$
<       This uses |/\@!| to ascertain that "VIM" does NOT match in any
        position after the first "VIM".
        Searching from the end of the file backwards is easier!

                                                */\%V*
\%V     Match inside the Visual area.  When Visual mode has already been
        stopped match in the area that |gv| would reselect.
        This is a |/zero-width| match.  To make sure the whole pattern is
        inside the Visual area put it at the start and end of the pattern,
        e.g.: >
                /\%Vfoo.*bar\%V
<       Only works for the current buffer.

                                                */\%#* *cursor-position*
\%#     Matches with the cursor position.  Only works when matching in a
        buffer displayed in a window. {not in Vi}
        WARNING: When the cursor is moved after the pattern was used, the
        result becomes invalid.  Vim doesn't automatically update the matches.
        This is especially relevant for syntax highlighting and 'hlsearch'.
        In other words: When the cursor moves the display isn't updated for
        this change.  An update is done for lines which are changed (the whole
        line is updated) or when using the |CTRL-L| command (the whole screen
        is updated).  Example, to highlight the word under the cursor: >
                /\k*\%#\k*
<       When 'hlsearch' is set and you move the cursor around and make changes
        this will clearly show when the match is updated or not.

                                                */\%'m* */\%<'m* */\%>'m*
\%'m    Matches with the position of mark m.
\%<'m   Matches before the position of mark m.
\%>'m   Matches after the position of mark m.
        Example, to highlight the text from mark 's to 'e: >
                /.\%>'s.*\%<'e..
<       Note that two dots are required to include mark 'e in the match.  That
        is because "\%<'e" matches at the character before the 'e mark, and
        since it's a |/zero-width| match it doesn't include that character.
        {not in Vi}
        WARNING: When the mark is moved after the pattern was used, the result
        becomes invalid.  Vim doesn't automatically update the matches.
        Similar to moving the cursor for "\%#" |/\%#|.

                                                */\%l* */\%>l* */\%<l*
\%23l   Matches in a specific line.
\%<23l  Matches above a specific line (lower line number).
\%>23l  Matches below a specific line (higher line number).
        These three can be used to match specific lines in a buffer.  The "23"
        can be any line number.  The first line is 1. {not in Vi}
        WARNING: When inserting or deleting lines Vim does not automatically
        update the matches.  This means Syntax highlighting quickly becomes
        wrong.
        Example, to highlight the line where the cursor currently is: >
                :exe '/\%' . line(".") . 'l.*'
<       When 'hlsearch' is set and you move the cursor around and make changes
        this will clearly show when the match is updated or not.

                                                */\%c* */\%>c* */\%<c*
\%23c   Matches in a specific column.
\%<23c  Matches before a specific column.
\%>23c  Matches after a specific column.
        These three can be used to match specific columns in a buffer or
        string.  The "23" can be any column number.  The first column is 1.
        Actually, the column is the byte number (thus it's not exactly right
        for multi-byte characters).  {not in Vi}
        WARNING: When inserting or deleting text Vim does not automatically
        update the matches.  This means Syntax highlighting quickly becomes
        wrong.
        Example, to highlight the column where the cursor currently is: >
                :exe '/\%' . col(".") . 'c'
<       When 'hlsearch' is set and you move the cursor around and make changes
        this will clearly show when the match is updated or not.
        Example for matching a single byte in column 44: >
                /\%>43c.\%<46c
<       Note that "\%<46c" matches in column 45 when the "." matches a byte in
        column 44.
                                                */\%v* */\%>v* */\%<v*
\%23v   Matches in a specific virtual column.
\%<23v  Matches before a specific virtual column.
\%>23v  Matches after a specific virtual column.
        These three can be used to match specific virtual columns in a buffer
        or string.  When not matching with a buffer in a window, the option
        values of the current window are used (e.g., 'tabstop').
        The "23" can be any column number.  The first column is 1.
        Note that some virtual column positions will never match, because they
        are halfway through a tab or other character that occupies more than
        one screen character.  {not in Vi}
        WARNING: When inserting or deleting text Vim does not automatically
        update highlighted matches.  This means Syntax highlighting quickly
        becomes wrong.
        Example, to highlight all the characters after virtual column 72: >
                /\%>72v.*
<       When 'hlsearch' is set and you move the cursor around and make changes
        this will clearly show when the match is updated or not.
        To match the text up to column 17: >
                /.*\%17v
<       Column 17 is not included, because that's where the "\%17v" matches,
        and since this is a |/zero-width| match, column 17 isn't included in
        the match.  This does the same: >
                /.*\%<18v
<

Character classes: {not in Vi}
\i      identifier character (see 'isident' option)     */\i*
\I      like "\i", but excluding digits                 */\I*
\k      keyword character (see 'iskeyword' option)      */\k*
\K      like "\k", but excluding digits                 */\K*
\f      file name character (see 'isfname' option)      */\f*
\F      like "\f", but excluding digits                 */\F*
\p      printable character (see 'isprint' option)      */\p*
\P      like "\p", but excluding digits                 */\P*

NOTE: the above also work for multi-byte characters.  The ones below only
match ASCII characters, as indicated by the range.

                                                *whitespace* *white-space*
\s      whitespace character: <Space> and <Tab>         */\s*
\S      non-whitespace character; opposite of \s        */\S*
\d      digit:                          [0-9]           */\d*
\D      non-digit:                      [^0-9]          */\D*
\x      hex digit:                      [0-9A-Fa-f]     */\x*
\X      non-hex digit:                  [^0-9A-Fa-f]    */\X*
\o      octal digit:                    [0-7]           */\o*
\O      non-octal digit:                [^0-7]          */\O*
\w      word character:                 [0-9A-Za-z_]    */\w*
\W      non-word character:             [^0-9A-Za-z_]   */\W*
\h      head of word character:         [A-Za-z_]       */\h*
\H      non-head of word character:     [^A-Za-z_]      */\H*
\a      alphabetic character:           [A-Za-z]        */\a*
\A      non-alphabetic character:       [^A-Za-z]       */\A*
\l      lowercase character:            [a-z]           */\l*
\L      non-lowercase character:        [^a-z]          */\L*
\u      uppercase character:            [A-Z]           */\u*
\U      non-uppercase character         [^A-Z]          */\U*

        NOTE: Using the atom is faster than the [] form.

        NOTE: 'ignorecase', "\c" and "\C" are not used by character classes.

                        */\_* *E63* */\_i* */\_I* */\_k* */\_K* */\_f* */\_F*
                        */\_p* */\_P* */\_s* */\_S* */\_d* */\_D* */\_x* */\_X*
                        */\_o* */\_O* */\_w* */\_W* */\_h* */\_H* */\_a* */\_A*
                        */\_l* */\_L* */\_u* */\_U*
\_x     Where "x" is any of the characters above: The character class with
        end-of-line added
(end of character classes)

\e      matches <Esc>                                   */\e*
\t      matches <Tab>                                   */\t*
\r      matches <CR>                                    */\r*
\b      matches <BS>                                    */\b*
\n      matches an end-of-line                          */\n*
        When matching in a string instead of buffer text a literal newline
        character is matched.

~       matches the last given substitute string        */~* */\~*

\(\)    A pattern enclosed by escaped parentheses.      */\(* */\(\)* */\)*
        E.g., "\(^a\)" matches 'a' at the start of a line.  *E51* *E54* *E55*

\1      Matches the same string that was matched by     */\1* *E65*
        the first sub-expression in \( and \). {not in Vi}
        Example: "\([a-z]\).\1" matches "ata", "ehe", "tot", etc.
\2      Like "\1", but uses second sub-expression,      */\2*
   ...                                                  */\3*
\9      Like "\1", but uses ninth sub-expression.       */\9*
        Note: The numbering of groups is done based on which "\(" comes first
        in the pattern (going left to right), NOT based on what is matched
        first.

\%(\)   A pattern enclosed by escaped parentheses.      */\%(\)* */\%(* *E53*
        Just like \(\), but without counting it as a sub-expression.  This
        allows using more groups and it's a little bit faster.
        {not in Vi}

x       A single character, with no special meaning, matches itself

                                                        */\* */\\*
\x      A backslash followed by a single character, with no special meaning,
        is reserved for future expansions

[]      (with 'nomagic': \[])           */[]* */\[]* */\_[]* */collection*
\_[]
        A collection.  This is a sequence of characters enclosed in brackets.
        It matches any single character in the collection.
        Example         matches ~
        [xyz]           any 'x', 'y' or 'z'
        [a-zA-Z]$       any alphabetic character at the end of a line
        \c[a-z]$        same
                                                                */[\n]*
        With "\_" prepended the collection also includes the end-of-line.
        The same can be done by including "\n" in the collection.  The
        end-of-line is also matched when the collection starts with "^"!  Thus
        "\_[^ab]" matches the end-of-line and any character but "a" and "b".
        This makes it Vi compatible: Without the "\_" or "\n" the collection
        does not match an end-of-line.
                                                                *E769*
        When the ']' is not there Vim will not give an error message but
        assume no collection is used.  Useful to search for '['.  However, you
        do get E769 for internal searching.

        If the sequence begins with "^", it matches any single character NOT
        in the collection: "[^xyz]" matches anything but 'x', 'y' and 'z'.
        - If two characters in the sequence are separated by '-', this is
          shorthand for the full list of ASCII characters between them.  E.g.,
          "[0-9]" matches any decimal digit.
        - A character class expression is evaluated to the set of characters
          belonging to that character class.  The following character classes
          are supported:
                          Name          Contents ~
*[:alnum:]*               [:alnum:]     letters and digits
*[:alpha:]*               [:alpha:]     letters
*[:blank:]*               [:blank:]     space and tab characters
*[:cntrl:]*               [:cntrl:]     control characters
*[:digit:]*               [:digit:]     decimal digits
*[:graph:]*               [:graph:]     printable characters excluding space
*[:lower:]*               [:lower:]     lowercase letters (all letters when
                                        'ignorecase' is used)
*[:print:]*               [:print:]     printable characters including space
*[:punct:]*               [:punct:]     punctuation characters
*[:space:]*               [:space:]     whitespace characters
*[:upper:]*               [:upper:]     uppercase letters (all letters when
                                        'ignorecase' is used)
*[:xdigit:]*              [:xdigit:]    hexadecimal digits
*[:return:]*              [:return:]    the <CR> character
*[:tab:]*                 [:tab:]       the <Tab> character
*[:escape:]*              [:escape:]    the <Esc> character
*[:backspace:]*           [:backspace:] the <BS> character
          The brackets in character class expressions are additional to the
          brackets delimiting a collection.  For example, the following is a
          plausible pattern for a UNIX filename: "[-./[:alnum:]_~]\+" That is,
          a list of at least one character, each of which is either '-', '.',
          '/', alphabetic, numeric, '_' or '~'.
          These items only work for 8-bit characters.
                                                        */[[=* *[==]*
        - An equivalence class.  This means that characters are matched that
          have almost the same meaning, e.g., when ignoring accents.  The form
          is:
                [=a=]
          Currently this is only implemented for latin1.  Also works for the
          latin1 characters in utf-8 and latin9.
                                                        */[[.* *[..]*
        - A collation element.  This currently simply accepts a single
          character in the form:
                [.a.]
                                                          */\]*
        - To include a literal ']', '^', '-' or '\' in the collection, put a
          backslash before it: "[xyz\]]", "[\^xyz]", "[xy\-z]" and "[xyz\\]".
          (Note: POSIX does not support the use of a backslash this way).  For
          ']' you can also make it the first character (following a possible
          "^"):  "[]xyz]" or "[^]xyz]" {not in Vi}.
          For '-' you can also make it the first or last character: "[-xyz]",
          "[^-xyz]" or "[xyz-]".  For '\' you can also let it be followed by
          any character that's not in "^]-\bertn".  "[\xyz]" matches '\', 'x',
          'y' and 'z'.  It's better to use "\\" though, future expansions may
          use other characters after '\'.
        - The following translations are accepted when the 'l' flag is not
          included in 'cpoptions' {not in Vi}:
                \e      <Esc>
                \t      <Tab>
                \r      <CR>    (NOT end-of-line!)
                \b      <BS>
                \n      line break, see above |/[\n]|
                \d123   decimal number of character
                \o40    octal number of character up to 0377
                \x20    hexadecimal number of character up to 0xff
                \u20AC  hex. number of multibyte character up to 0xffff
                \U1234  hex. number of multibyte character up to 0xffffffff
          NOTE: The other backslash codes mentioned above do not work inside
          []!
        - Matching with a collection can be slow, because each character in
          the text has to be compared with each character in the collection.
          Use one of the other atoms above when possible.  Example: "\d" is
          much faster than "[0-9]" and matches the same characters.

                                                */\%[]* *E69* *E70* *E369*
\%[]    A sequence of optionally matched atoms.  This always matches.
        It matches as much of the list of atoms it contains as possible.  Thus
        it stops at the first atom that doesn't match.  For example: >
                /r\%[ead]
<       matches "r", "re", "rea" or "read".  The longest that matches is used.
        To match the Ex command "function", where "fu" is required and
        "nction" is optional, this would work: >
                /\<fu\%[nction]\>
<       The end-of-word atom "\>" is used to avoid matching "fu" in "full".
        It gets more complicated when the atoms are not ordinary characters.
        You don't often have to use it, but it is possible.  Example: >
                /\<r\%[[eo]ad]\>
<       Matches the words "r", "re", "ro", "rea", "roa", "read" and "road".
        There can be no \(\), \%(\) or \z(\) items inside the [] and \%[] does
        not nest.
        To include a "[" use "[[]" and for "]" use []]", e.g.,: >
                /index\%[[[]0[]]]
<       matches "index" "index[", "index[0" and "index[0]".
        {not available when compiled without the +syntax feature}

                                */\%d* */\%x* */\%o* */\%u* */\%U* *E678*

\%d123  Matches the character specified with a decimal number.  Must be
        followed by a non-digit.
\%o40   Matches the character specified with an octal number up to 0377.
        Numbers below 040 must be followed by a non-octal digit or a non-digit.
\%x2a   Matches the character specified with up to two hexadecimal characters.
\%u20AC Matches the character specified with up to four hexadecimal
        characters.
\%U1234abcd     Matches the character specified with up to eight hexadecimal
        characters.

==============================================================================
7. Ignoring case in a pattern                                   */ignorecase*

If the 'ignorecase' option is on, the case of normal letters is ignored.
'smartcase' can be set to ignore case when the pattern contains lowercase
letters only.
                                                        */\c* */\C*
When "\c" appears anywhere in the pattern, the whole pattern is handled like
'ignorecase' is on.  The actual value of 'ignorecase' and 'smartcase' is
ignored.  "\C" does the opposite: Force matching case for the whole pattern.
{only Vim supports \c and \C}
Note that 'ignorecase', "\c" and "\C" are not used for the character classes.

Examples:
      pattern   'ignorecase'  'smartcase'       matches ~
        foo       off           -               foo
        foo       on            -               foo Foo FOO
        Foo       on            off             foo Foo FOO
        Foo       on            on                  Foo
        \cfoo     -             -               foo Foo FOO
        foo\C     -             -               foo

Technical detail:                               *NL-used-for-Nul*
<Nul> characters in the file are stored as <NL> in memory.  In the display
they are shown as "^@".  The translation is done when reading and writing
files.  To match a <Nul> with a search pattern you can just enter CTRL-@ or
"CTRL-V 000".  This is probably just what you expect.  Internally the
character is replaced with a <NL> in the search pattern.  What is unusual is
that typing CTRL-V CTRL-J also inserts a <NL>, thus also searches for a <Nul>
in the file.  {Vi cannot handle <Nul> characters in the file at all}

                                                *CR-used-for-NL*
When 'fileformat' is "mac", <NL> characters in the file are stored as <CR>
characters internally.  In the text they are shown as "^J".  Otherwise this
works similar to the usage of <NL> for a <Nul>.

When working with expression evaluation, a <NL> character in the pattern
matches a <NL> in the string.  The use of "\n" (backslash n) to match a <NL>
doesn't work there, it only works to match text in the buffer.

                                                *pattern-multi-byte*
Patterns will also work with multi-byte characters, mostly as you would
expect.  But invalid bytes may cause trouble, a pattern with an invalid byte
will probably never match.

==============================================================================
8. Composing characters                                 *patterns-composing*

                                                        */\Z*
When "\Z" appears anywhere in the pattern, composing characters are ignored.
Thus only the base characters need to match, the composing characters may be
different and the number of composing characters may differ.  Only relevant
when 'encoding' is "utf-8".

When a composing character appears at the start of the pattern of after an
item that doesn't include the composing character, a match is found at any
character that includes this composing character.

When using a dot and a composing character, this works the same as the
composing character by itself, except that it doesn't matter what comes before
this.

The order of composing characters matters, even though changing the order
doesn't change what a character looks like.  This may change in the future.

==============================================================================
9. Compare with Perl patterns                           *perl-patterns*

Vim's regexes are most similar to Perl's, in terms of what you can do.  The
difference between them is mostly just notation;  here's a summary of where
they differ:

Capability                      in Vimspeak     in Perlspeak ~
----------------------------------------------------------------
force case insensitivity        \c              (?i)
force case sensitivity          \C              (?-i)
backref-less grouping           \%(atom\)       (?:atom)
conservative quantifiers        \{-n,m}         *?, +?, ??, {}?
0-width match                   atom\@=         (?=atom)
0-width non-match               atom\@!         (?!atom)
0-width preceding match         atom\@<=        (?<=atom)
0-width preceding non-match     atom\@<!        (?<!atom)
match without retry             atom\@>         (?>atom)

Vim and Perl handle newline characters inside a string a bit differently:

In Perl, ^ and $ only match at the very beginning and end of the text,
by default, but you can set the 'm' flag, which lets them match at
embedded newlines as well.  You can also set the 's' flag, which causes
a . to match newlines as well.  (Both these flags can be changed inside
a pattern using the same syntax used for the i flag above, BTW.)

On the other hand, Vim's ^ and $ always match at embedded newlines, and
you get two separate atoms, \%^ and \%$, which only match at the very
start and end of the text, respectively.  Vim solves the second problem
by giving you the \_ "modifier":  put it in front of a . or a character
class, and they will match newlines as well.

Finally, these constructs are unique to Perl:
- execution of arbitrary code in the regex:  (?{perl code})
- conditional expressions:  (?(condition)true-expr|false-expr)

...and these are unique to Vim:
- changing the magic-ness of a pattern:  \v \V \m \M
   (very useful for avoiding backslashitis)
- sequence of optionally matching atoms:  \%[atoms]
- \& (which is to \| what "and" is to "or";  it forces several branches
   to match at one spot)
- matching lines/columns by number:  \%5l \%5c \%5v
- setting the start and end of the match:  \zs \ze

==============================================================================
10. Highlighting matches                                *match-highlight*

                                                        *:mat* *:match*
:mat[ch] {group} /{pattern}/
                Define a pattern to highlight in the current window.  It will
                be highlighted with {group}.  Example: >
                        :highlight MyGroup ctermbg=green guibg=green
                        :match MyGroup /TODO/
<               Instead of // any character can be used to mark the start and
                end of the {pattern}.  Watch out for using special characters,
                such as '"' and '|'.

                {group} must exist at the moment this command is executed.

                The {group} highlighting still applies when a character is
                to be highlighted for 'hlsearch', as the highlighting for
                matches is given higher priority than that of 'hlsearch'.
                Syntax highlighting (see 'syntax') is also overruled by
                matches.

                Note that highlighting the last used search pattern with
                'hlsearch' is used in all windows, while the pattern defined
                with ":match" only exists in the current window.  It is kept
                when switching to another buffer.

                'ignorecase' does not apply, use |/\c| in the pattern to
                ignore case.  Otherwise case is not ignored.

                'redrawtime' defines the maximum time searched for pattern
                matches.

                When matching end-of-line and Vim redraws only part of the
                display you may get unexpected results.  That is because Vim
                looks for a match in the line where redrawing starts.

                Also see |matcharg()| and |getmatches()|. The former returns
                the highlight group and pattern of a previous |:match|
                command.  The latter returns a list with highlight groups and
                patterns defined by both |matchadd()| and |:match|.

                Highlighting matches using |:match| are limited to three
                matches (aside from |:match|, |:2match| and |:3match|are
                available). |matchadd()| does not have this limitation and in
                addition makes it possible to prioritize matches.

                Another example, which highlights all characters in virtual
                column 72 and more: >
                        :highlight rightMargin term=bold ctermfg=blue guifg=blue
                        :match rightMargin /.\%>72v/
<               To highlight all character that are in virtual column 7: >
                        :highlight col8 ctermbg=grey guibg=grey
                        :match col8 /\%<8v.\%>7v/
<               Note the use of two items to also match a character that
                occupies more than one virtual column, such as a TAB.

:mat[ch]
:mat[ch] none
                Clear a previously defined match pattern.


:2mat[ch] {group} /{pattern}/                                   *:2match*
:2mat[ch]
:2mat[ch] none
:3mat[ch] {group} /{pattern}/                                   *:3match*
:3mat[ch]
:3mat[ch] none
                Just like |:match| above, but set a separate match.  Thus
                there can be three matches active at the same time.  The match
                with the lowest number has priority if several match at the
                same position.
                The ":3match" command is used by the |matchparen| plugin.  You
                are suggested to use ":match" for manual matching and
                ":2match" for another plugin.


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