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1=head1 NAME
2
3perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
4
5=head1 DESCRIPTION
6
7The top level documentation about Perl regular expressions
8is found in L<perlre>.
9
10This document describes all backslash and escape sequences. After
11explaining the role of the backslash, it lists all the sequences that have
12a special meaning in Perl regular expressions (in alphabetical order),
13then describes each of them.
14
15Most sequences are described in detail in different documents; the primary
16purpose of this document is to have a quick reference guide describing all
17backslash and escape sequences.
18
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19=head2 The backslash
20
21In a regular expression, the backslash can perform one of two tasks:
22it either takes away the special meaning of the character following it
23(for instance, C<\|> matches a vertical bar, it's not an alternation),
24or it is the start of a backslash or escape sequence.
25
26The rules determining what it is are quite simple: if the character
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27following the backslash is an ASCII punctuation (non-word) character (that is,
28anything that is not a letter, digit or underscore), then the backslash just
29takes away the special meaning (if any) of the character following it.
30
31If the character following the backslash is an ASCII letter or an ASCII digit,
32then the sequence may be special; if so, it's listed below. A few letters have
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33not been used yet, so escaping them with a backslash doesn't change them to be
34special. A future version of Perl may assign a special meaning to them, so if
35you have warnings turned on, Perl will issue a warning if you use such a
36sequence. [1].
8a118206 37
e2cb52ee 38It is however guaranteed that backslash or escape sequences never have a
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39punctuation character following the backslash, not now, and not in a future
40version of Perl 5. So it is safe to put a backslash in front of a non-word
41character.
42
43Note that the backslash itself is special; if you want to match a backslash,
44you have to escape the backslash with a backslash: C</\\/> matches a single
45backslash.
46
47=over 4
48
49=item [1]
50
51There is one exception. If you use an alphanumerical character as the
52delimiter of your pattern (which you probably shouldn't do for readability
53reasons), you will have to escape the delimiter if you want to match
54it. Perl won't warn then. See also L<perlop/Gory details of parsing
55quoted constructs>.
56
57=back
58
59
60=head2 All the sequences and escapes
61
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62Those not usable within a bracketed character class (like C<[\da-z]>) are marked
63as C<Not in [].>
64
8a118206 65 \000 Octal escape sequence.
df225385 66 \1 Absolute backreference. Not in [].
8a118206 67 \a Alarm or bell.
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68 \A Beginning of string. Not in [].
69 \b Word/non-word boundary. (Backspace in []).
70 \B Not a word/non-word boundary. Not in [].
4948b50f 71 \cX Control-X
df225385 72 \C Single octet, even under UTF-8. Not in [].
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73 \d Character class for digits.
74 \D Character class for non-digits.
75 \e Escape character.
df225385 76 \E Turn off \Q, \L and \U processing. Not in [].
8a118206 77 \f Form feed.
f822d0dd 78 \g{}, \g1 Named, absolute or relative backreference. Not in []
df225385 79 \G Pos assertion. Not in [].
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80 \h Character class for horizontal whitespace.
81 \H Character class for non horizontal whitespace.
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82 \k{}, \k<>, \k'' Named backreference. Not in [].
83 \K Keep the stuff left of \K. Not in [].
84 \l Lowercase next character. Not in [].
85 \L Lowercase till \E. Not in [].
8a118206 86 \n (Logical) newline character.
b3b85878 87 \N Any character but newline. Experimental. Not in [].
e526e8bb 88 \N{} Named or numbered (Unicode) character.
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89 \p{}, \pP Character with the given Unicode property.
90 \P{}, \PP Character without the given Unicode property.
df225385 91 \Q Quotemeta till \E. Not in [].
8a118206 92 \r Return character.
df225385 93 \R Generic new line. Not in [].
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94 \s Character class for whitespace.
95 \S Character class for non whitespace.
8a118206 96 \t Tab character.
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97 \u Titlecase next character. Not in [].
98 \U Uppercase till \E. Not in [].
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99 \v Character class for vertical whitespace.
100 \V Character class for non vertical whitespace.
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101 \w Character class for word characters.
102 \W Character class for non-word characters.
103 \x{}, \x00 Hexadecimal escape sequence.
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104 \X Unicode "extended grapheme cluster". Not in [].
105 \z End of string. Not in [].
106 \Z End of string. Not in [].
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107
108=head2 Character Escapes
109
110=head3 Fixed characters
111
e2cb52ee 112A handful of characters have a dedicated I<character escape>. The following
58151fe4 113table shows them, along with their ASCII code points (in decimal and hex),
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114their ASCII name, the control escape on ASCII platforms and a short
115description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
8a118206 116
4948b50f 117 Seq. Code Point ASCII Cntrl Description.
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118 Dec Hex
119 \a 7 07 BEL \cG alarm or bell
120 \b 8 08 BS \cH backspace [1]
121 \e 27 1B ESC \c[ escape character
122 \f 12 0C FF \cL form feed
123 \n 10 0A LF \cJ line feed [2]
124 \r 13 0D CR \cM carriage return
125 \t 9 09 TAB \cI tab
126
127=over 4
128
129=item [1]
130
301ba1af 131C<\b> is the backspace character only inside a character class. Outside a
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132character class, C<\b> is a word/non-word boundary.
133
134=item [2]
135
136C<\n> matches a logical newline. Perl will convert between C<\n> and your
f6993e9e 137OS's native newline character when reading from or writing to text files.
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138
139=back
140
141=head4 Example
142
143 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
144
145=head3 Control characters
146
147C<\c> is used to denote a control character; the character following C<\c>
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148determines the value of the construct. For example the value of C<\cA> is
149C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
150The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
151list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
152L<perlebcdic/OPERATOR DIFFERENCES>.
153
154Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
155string) is not valid. The backslash must be followed by another character.
156That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
157
158To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
159C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
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160
161Mnemonic: I<c>ontrol character.
162
163=head4 Example
164
165 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
166
e526e8bb 167=head3 Named or numbered characters
8a118206 168
f6993e9e 169Unicode characters have a Unicode name and numeric ordinal value. Use the
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170C<\N{}> construct to specify a character by either of these values.
171
172To specify by name, the name of the character goes between the curly braces.
173In this case, you have to C<use charnames> to load the Unicode names of the
174characters, otherwise Perl will complain.
175
176To specify by Unicode ordinal number, use the form
177C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
178hexadecimal that gives the ordinal number that Unicode has assigned to the
179desired character. It is customary (but not required) to use leading zeros to
180pad the number to 4 digits. Thus C<\N{U+0041}> means
181C<Latin Capital Letter A>, and you will rarely see it written without the two
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182leading zeros. C<\N{U+0041}> means "A" even on EBCDIC machines (where the
183ordinal value of "A" is not 0x41).
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184
185It is even possible to give your own names to characters, and even to short
186sequences of characters. For details, see L<charnames>.
8a118206 187
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188(There is an expanded internal form that you may see in debug output:
189C<\N{U+I<wide hex character>.I<wide hex character>...}>.
190The C<...> means any number of these I<wide hex character>s separated by dots.
191This represents the sequence formed by the characters. This is an internal
192form only, subject to change, and you should not try to use it yourself.)
193
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194Mnemonic: I<N>amed character.
195
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196Note that a character that is expressed as a named or numbered character is
197considered as a character without special meaning by the regex engine, and will
198match "as is".
df225385 199
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200=head4 Example
201
202 use charnames ':full'; # Loads the Unicode names.
203 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
204
205 use charnames 'Cyrillic'; # Loads Cyrillic names.
206 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
207
208=head3 Octal escapes
209
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210Octal escapes consist of a backslash followed by three octal digits
211matching the code point of the character you want to use. (In some contexts,
212two or even one octal digits are also accepted, sometimes with a warning.) This
213allows for 512 characters (C<\000> up to C<\777>) that can be expressed this
fa1639c5 214way. Enough in pre-Unicode days,
ce7b6f06 215but most Unicode characters cannot be escaped this way.
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216
217Note that a character that is expressed as an octal escape is considered
218as a character without special meaning by the regex engine, and will match
219"as is".
220
58151fe4 221=head4 Examples (assuming an ASCII platform)
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222
223 $str = "Perl";
224 $str =~ /\120/; # Match, "\120" is "P".
f822d0dd 225 $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once
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226 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
227
228=head4 Caveat
229
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230Octal escapes potentially clash with old-style backreferences (see L</Absolute
231referencing> below). They both consist of a backslash followed by numbers. So
232Perl has to use heuristics to determine whether it is a backreference or an
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233octal escape. You can avoid ambiguity by using the C<\g> form for
234backreferences, and by beginning octal escapes with a "0". (Since octal
235escapes are 3 digits, this latter method works only up to C<\077>.) In the
236absence of C<\g>, Perl uses the following rules:
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237
238=over 4
239
240=item 1
241
353c6505 242If the backslash is followed by a single digit, it's a backreference.
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243
244=item 2
245
246If the first digit following the backslash is a 0, it's an octal escape.
247
248=item 3
249
f6993e9e 250If the number following the backslash is N (in decimal), and Perl already has
8a118206 251seen N capture groups, Perl will consider this to be a backreference.
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252Otherwise, it will consider it to be an octal escape. Note that if N has more
253than three digits, Perl only takes the first three for the octal escape;
254the rest are matched as is.
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255
256 my $pat = "(" x 999;
257 $pat .= "a";
258 $pat .= ")" x 999;
259 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
260 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
261 # and \1000 is seen as \100 (a '@') and a '0'.
262
263=back
264
265=head3 Hexadecimal escapes
266
58151fe4 267Hexadecimal escapes start with C<\x> and are then either followed by a
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268two digit hexadecimal number, or a hexadecimal number of arbitrary length
269surrounded by curly braces. The hexadecimal number is the code point of
270the character you want to express.
271
272Note that a character that is expressed as a hexadecimal escape is considered
273as a character without special meaning by the regex engine, and will match
274"as is".
275
276Mnemonic: heI<x>adecimal.
277
9f5650a8 278=head4 Examples (assuming an ASCII platform)
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279
280 $str = "Perl";
281 $str =~ /\x50/; # Match, "\x50" is "P".
f822d0dd 282 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
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283 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
284
285 /\x{2603}\x{2602}/ # Snowman with an umbrella.
286 # The Unicode character 2603 is a snowman,
287 # the Unicode character 2602 is an umbrella.
288 /\x{263B}/ # Black smiling face.
289 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
290
291=head2 Modifiers
292
293A number of backslash sequences have to do with changing the character,
294or characters following them. C<\l> will lowercase the character following
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295it, while C<\u> will uppercase (or, more accurately, titlecase) the
296character following it. (They perform similar functionality as the
297functions C<lcfirst> and C<ucfirst>).
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298
299To uppercase or lowercase several characters, one might want to use
300C<\L> or C<\U>, which will lowercase/uppercase all characters following
e2cb52ee 301them, until either the end of the pattern, or the next occurrence of
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302C<\E>, whatever comes first. They perform similar functionality as the
303functions C<lc> and C<uc> do.
304
305C<\Q> is used to escape all characters following, up to the next C<\E>
306or the end of the pattern. C<\Q> adds a backslash to any character that
307isn't a letter, digit or underscore. This will ensure that any character
308between C<\Q> and C<\E> is matched literally, and will not be interpreted
309by the regexp engine.
310
311Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
312
313=head4 Examples
314
315 $sid = "sid";
316 $greg = "GrEg";
317 $miranda = "(Miranda)";
318 $str =~ /\u$sid/; # Matches 'Sid'
319 $str =~ /\L$greg/; # Matches 'greg'
320 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
321 # had been written as /\(Miranda\)/
322
323=head2 Character classes
324
325Perl regular expressions have a large range of character classes. Some of
326the character classes are written as a backslash sequence. We will briefly
327discuss those here; full details of character classes can be found in
328L<perlrecharclass>.
329
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330C<\w> is a character class that matches any single I<word> character (letters,
331digits, underscore). C<\d> is a character class that matches any decimal digit,
418e7b04 332while the character class C<\s> matches any whitespace character.
99d59c4d 333New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
418e7b04 334and vertical whitespace characters.
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335
336The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
337character classes that match any character that isn't a word character,
418e7b04 338digit, whitespace, horizontal whitespace nor vertical whitespace.
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339
340Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
341
342=head3 Unicode classes
343
344C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
345match a character that matches the given Unicode property; properties
346include things like "letter", or "thai character". Capitalizing the
347sequence to C<\PP> and C<\P{Property}> make the sequence match a character
348that doesn't match the given Unicode property. For more details, see
4948b50f 349L<perlrecharclass/Backslash sequences> and
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350L<perlunicode/Unicode Character Properties>.
351
352Mnemonic: I<p>roperty.
353
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354=head2 Referencing
355
356If capturing parenthesis are used in a regular expression, we can refer
357to the part of the source string that was matched, and match exactly the
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358same thing. There are three ways of referring to such I<backreference>:
359absolutely, relatively, and by name.
360
361=for later add link to perlrecapture
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362
363=head3 Absolute referencing
364
c27a5cfe 365Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
d8b950dc 366is a positive (unsigned) decimal number of any length is an absolute reference
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367to a capturing group.
368
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369I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
370been matched by that set of parentheses. Thus C<\g1> refers to the first
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371capture group in the regex.
372
373The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
374which avoids ambiguity when building a regex by concatenating shorter
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375strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
376C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
377probably not what you intended.
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378
379In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
380least I<N> capturing groups, or else I<N> will be considered an octal escape
381(but something like C<\18> is the same as C<\0018>, that is the octal escape
382C<"\001"> followed by a literal digit C<"8">).
383
384Mnemonic: I<g>roup.
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385
386=head4 Examples
387
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388 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
389 /(\w+) \1/; # Same thing; written old-style
390 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
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391
392
393=head3 Relative referencing
394
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395C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
396be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
397C<\g{-I<N>}>.
8a118206 398
c27a5cfe 399The big advantage of this form is that it makes it much easier to write
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400patterns with references that can be interpolated in larger patterns,
401even if the larger pattern also contains capture groups.
402
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403=head4 Examples
404
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405 /(A) # Group 1
406 ( # Group 2
407 (B) # Group 3
408 \g{-1} # Refers to group 3 (B)
409 \g{-3} # Refers to group 1 (A)
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410 )
411 /x; # Matches "ABBA".
412
413 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
414 /$qr$qr/ # Matches 'ababcdcd'.
415
416=head3 Named referencing
417
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418C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
419named capture group, dispensing completely with having to think about capture
420buffer positions.
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421
422To be compatible with .Net regular expressions, C<\g{name}> may also be
423written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
424
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425To prevent any ambiguity, I<name> must not start with a digit nor contain a
426hyphen.
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427
428=head4 Examples
429
430 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
431 /(?<word>\w+) \k{word}/ # Same.
432 /(?<word>\w+) \k<word>/ # Same.
433 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
434 # Match a four letter palindrome (e.g. "ABBA")
435
436=head2 Assertions
437
ac036724 438Assertions are conditions that have to be true; they don't actually
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439match parts of the substring. There are six assertions that are written as
440backslash sequences.
441
442=over 4
443
444=item \A
445
446C<\A> only matches at the beginning of the string. If the C</m> modifier
447isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
448modifier is used, then C</^/> matches internal newlines, but the meaning
449of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
450of the string regardless whether the C</m> modifier is used.
451
452=item \z, \Z
453
454C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
455used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
456end of the string, or before the newline at the end of the string. If the
457C</m> modifier is used, then C</$/> matches at internal newlines, but the
458meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
459the end of the string (or just before a trailing newline) regardless whether
460the C</m> modifier is used.
461
462C<\z> is just like C<\Z>, except that it will not match before a trailing
463newline. C<\z> will only match at the end of the string - regardless of the
464modifiers used, and not before a newline.
465
466=item \G
467
468C<\G> is usually only used in combination with the C</g> modifier. If the
469C</g> modifier is used (and the match is done in scalar context), Perl will
470remember where in the source string the last match ended, and the next time,
471it will start the match from where it ended the previous time.
472
473C<\G> matches the point where the previous match ended, or the beginning
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474of the string if there was no previous match.
475
476=for later add link to perlremodifiers
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477
478Mnemonic: I<G>lobal.
479
480=item \b, \B
481
482C<\b> matches at any place between a word and a non-word character; C<\B>
483matches at any place between characters where C<\b> doesn't match. C<\b>
484and C<\B> assume there's a non-word character before the beginning and after
485the end of the source string; so C<\b> will match at the beginning (or end)
486of the source string if the source string begins (or ends) with a word
487character. Otherwise, C<\B> will match.
488
489Mnemonic: I<b>oundary.
490
491=back
492
493=head4 Examples
494
495 "cat" =~ /\Acat/; # Match.
496 "cat" =~ /cat\Z/; # Match.
497 "cat\n" =~ /cat\Z/; # Match.
498 "cat\n" =~ /cat\z/; # No match.
499
500 "cat" =~ /\bcat\b/; # Matches.
501 "cats" =~ /\bcat\b/; # No match.
502 "cat" =~ /\bcat\B/; # No match.
503 "cats" =~ /\bcat\B/; # Match.
504
505 while ("cat dog" =~ /(\w+)/g) {
506 print $1; # Prints 'catdog'
507 }
508 while ("cat dog" =~ /\G(\w+)/g) {
509 print $1; # Prints 'cat'
510 }
511
512=head2 Misc
513
514Here we document the backslash sequences that don't fall in one of the
515categories above. They are:
516
517=over 4
518
519=item \C
520
521C<\C> always matches a single octet, even if the source string is encoded
522in UTF-8 format, and the character to be matched is a multi-octet character.
523C<\C> was introduced in perl 5.6.
524
525Mnemonic: oI<C>tet.
526
527=item \K
528
99d59c4d 529This is new in perl 5.10.0. Anything that is matched left of C<\K> is
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530not included in C<$&> - and will not be replaced if the pattern is
531used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
532instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
533
534Mnemonic: I<K>eep.
535
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536=item \N
537
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538This is a new experimental feature in perl 5.12.0. It matches any character
539that is not a newline. It is a short-hand for writing C<[^\n]>, and is
540identical to the C<.> metasymbol, except under the C</s> flag, which changes
541the meaning of C<.>, but not C<\N>.
df225385 542
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543Note that C<\N{...}> can mean a
544L<named or numbered character|/Named or numbered characters>.
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545
546Mnemonic: Complement of I<\n>.
547
8a118206 548=item \R
6b46370c 549X<\R>
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550
551C<\R> matches a I<generic newline>, that is, anything that is considered
552a newline by Unicode. This includes all characters matched by C<\v>
418e7b04 553(vertical whitespace), and the multi character sequence C<"\x0D\x0A">
8a118206 554(carriage return followed by a line feed, aka the network newline, or
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555the newline used in Windows text files). C<\R> is equivalent to
556C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
557character, it cannot be put inside a bracketed character class; C</[\R]/> is an
558error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
8a118206 559
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560Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
561and more importantly because Unicode recommends such a regular expression
562metacharacter, and suggests C<\R> as the notation.
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563
564=item \X
6b46370c 565X<\X>
8a118206 566
0111a78f 567This matches a Unicode I<extended grapheme cluster>.
8a118206 568
10fdd326 569C<\X> matches quite well what normal (non-Unicode-programmer) usage
0111a78f 570would consider a single character. As an example, consider a G with some sort
c670e63a 571of diacritic mark, such as an arrow. There is no such single character in
df225385 572Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
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573UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
574were a single character.
10fdd326 575
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576Mnemonic: eI<X>tended Unicode character.
577
578=back
579
580=head4 Examples
581
582 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
583
f822d0dd 584 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
d8b950dc 585 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
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586
587 "\n" =~ /^\R$/; # Match, \n is a generic newline.
588 "\r" =~ /^\R$/; # Match, \r is a generic newline.
589 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
590
591 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
592
593=cut