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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
f0a2b745 65 \000 Octal escape sequence. See also \o{}.
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 [].
fb121860 88 \N{} Named or numbered (Unicode) character or sequence.
f0a2b745 89 \o{} Octal escape sequence.
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90 \p{}, \pP Character with the given Unicode property.
91 \P{}, \PP Character without the given Unicode property.
df225385 92 \Q Quotemeta till \E. Not in [].
8a118206 93 \r Return character.
df225385 94 \R Generic new line. Not in [].
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95 \s Character class for whitespace.
96 \S Character class for non whitespace.
8a118206 97 \t Tab character.
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98 \u Titlecase next character. Not in [].
99 \U Uppercase till \E. Not in [].
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100 \v Character class for vertical whitespace.
101 \V Character class for non vertical whitespace.
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102 \w Character class for word characters.
103 \W Character class for non-word characters.
104 \x{}, \x00 Hexadecimal escape sequence.
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105 \X Unicode "extended grapheme cluster". Not in [].
106 \z End of string. Not in [].
107 \Z End of string. Not in [].
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108
109=head2 Character Escapes
110
111=head3 Fixed characters
112
e2cb52ee 113A handful of characters have a dedicated I<character escape>. The following
58151fe4 114table shows them, along with their ASCII code points (in decimal and hex),
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115their ASCII name, the control escape on ASCII platforms and a short
116description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
8a118206 117
4948b50f 118 Seq. Code Point ASCII Cntrl Description.
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119 Dec Hex
120 \a 7 07 BEL \cG alarm or bell
121 \b 8 08 BS \cH backspace [1]
122 \e 27 1B ESC \c[ escape character
123 \f 12 0C FF \cL form feed
124 \n 10 0A LF \cJ line feed [2]
125 \r 13 0D CR \cM carriage return
126 \t 9 09 TAB \cI tab
127
128=over 4
129
130=item [1]
131
301ba1af 132C<\b> is the backspace character only inside a character class. Outside a
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133character class, C<\b> is a word/non-word boundary.
134
135=item [2]
136
137C<\n> matches a logical newline. Perl will convert between C<\n> and your
f6993e9e 138OS's native newline character when reading from or writing to text files.
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139
140=back
141
142=head4 Example
143
144 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
145
146=head3 Control characters
147
148C<\c> is used to denote a control character; the character following C<\c>
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149determines the value of the construct. For example the value of C<\cA> is
150C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
151The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
152list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
153L<perlebcdic/OPERATOR DIFFERENCES>.
154
155Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
156string) is not valid. The backslash must be followed by another character.
157That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
158
159To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
160C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
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161
162Mnemonic: I<c>ontrol character.
163
164=head4 Example
165
166 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
167
fb121860 168=head3 Named or numbered characters and character sequences
8a118206 169
f6993e9e 170Unicode characters have a Unicode name and numeric ordinal value. Use the
e526e8bb 171C<\N{}> construct to specify a character by either of these values.
fb121860 172Certain sequences of characters also have names.
e526e8bb 173
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174To specify by name, the name of the character or character sequence goes
175between the curly braces. In this case, you have to C<use charnames> to
176load the Unicode names of the characters, otherwise Perl will complain.
e526e8bb 177
835df198 178To specify a character by Unicode code point, use the form
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179C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
180hexadecimal that gives the ordinal number that Unicode has assigned to the
181desired character. It is customary (but not required) to use leading zeros to
182pad the number to 4 digits. Thus C<\N{U+0041}> means
835df198 183C<LATIN CAPITAL LETTER A>, and you will rarely see it written without the two
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184leading zeros. C<\N{U+0041}> means "A" even on EBCDIC machines (where the
185ordinal value of "A" is not 0x41).
e526e8bb 186
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187It is even possible to give your own names to characters and character
188sequences. For details, see L<charnames>.
8a118206 189
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190(There is an expanded internal form that you may see in debug output:
191C<\N{U+I<wide hex character>.I<wide hex character>...}>.
192The C<...> means any number of these I<wide hex character>s separated by dots.
193This represents the sequence formed by the characters. This is an internal
194form only, subject to change, and you should not try to use it yourself.)
195
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196Mnemonic: I<N>amed character.
197
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198Note that a character or character sequence that is expressed as a named
199or numbered character is considered as a character without special
200meaning by the regex engine, and will match "as is".
df225385 201
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202=head4 Example
203
204 use charnames ':full'; # Loads the Unicode names.
205 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
206
207 use charnames 'Cyrillic'; # Loads Cyrillic names.
208 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
209
210=head3 Octal escapes
211
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212There are two forms of octal escapes. Each is used to specify a character by
213its ordinal, specified in octal notation.
214
215One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
216represent one or more octal digits. It can be used for any Unicode character.
217
218It was introduced to avoid the potential problems with the other form,
219available in all Perls. That form consists of a backslash followed by three
220octal digits. One problem with this form is that it can look exactly like an
221old-style backreference (see
222L</Disambiguation rules between old-style octal escapes and backreferences>
223below.) You can avoid this by making the first of the three digits always a
224zero, but that makes \077 the largest ordinal unambiguously specifiable by this
225form.
226
227In some contexts, a backslash followed by two or even one octal digits may be
228interpreted as an octal escape, sometimes with a warning, and because of some
229bugs, sometimes with surprising results. Also, if you are creating a regex
c69ca1d4 230out of smaller snippets concatenated together, and you use fewer than three
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231digits, the beginning of one snippet may be interpreted as adding digits to the
232ending of the snippet before it. See L</Absolute referencing> for more
233discussion and examples of the snippet problem.
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234
235Note that a character that is expressed as an octal escape is considered
236as a character without special meaning by the regex engine, and will match
237"as is".
238
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239To summarize, the C<\o{}> form is always safe to use, and the other form is
240safe to use for ordinals up through \077 when you use exactly three digits to
241specify them.
8a118206 242
f0a2b745 243Mnemonic: I<0>ctal or I<o>ctal.
8a118206 244
f0a2b745 245=head4 Examples (assuming an ASCII platform)
8a118206 246
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247 $str = "Perl";
248 $str =~ /\o{120}/; # Match, "\120" is "P".
249 $str =~ /\120/; # Same.
250 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
251 $str =~ /\120+/; # Same.
252 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
253 /\o{23073}/ # Black foreground, white background smiling face.
254 /\o{4801234567}/ # Raises a warning, and yields chr(4)
255
256=head4 Disambiguation rules between old-style octal escapes and backreferences
257
258Octal escapes of the C<\000> form outside of bracketed character classes
259potentially clash with old-style backreferences. (see L</Absolute referencing>
260below). They both consist of a backslash followed by numbers. So Perl has to
261use heuristics to determine whether it is a backreference or an octal escape.
262Perl uses the following rules to disambiguate:
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263
264=over 4
265
266=item 1
267
353c6505 268If the backslash is followed by a single digit, it's a backreference.
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269
270=item 2
271
272If the first digit following the backslash is a 0, it's an octal escape.
273
274=item 3
275
f6993e9e 276If the number following the backslash is N (in decimal), and Perl already has
8a118206 277seen N capture groups, Perl will consider this to be a backreference.
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278Otherwise, it will consider it to be an octal escape. Note that if N has more
279than three digits, Perl only takes the first three for the octal escape;
280the rest are matched as is.
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281
282 my $pat = "(" x 999;
283 $pat .= "a";
284 $pat .= ")" x 999;
285 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
286 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
f0a2b745 287 # and \1000 is seen as \100 (a '@') and a '0'
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288
289=back
290
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291You can the force a backreference interpretation always by using the C<\g{...}>
292form. You can the force an octal interpretation always by using the C<\o{...}>
293form, or for numbers up through \077 (= 63 decimal), by using three digits,
294beginning with a "0".
295
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296=head3 Hexadecimal escapes
297
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298Like octal escapes, there are two forms of hexadecimal escapes, but both start
299with the same thing, C<\x>. This is followed by either exactly two hexadecimal
300digits forming a number, or a hexadecimal number of arbitrary length surrounded
301by curly braces. The hexadecimal number is the code point of the character you
302want to express.
8a118206 303
f0a2b745 304Note that a character that is expressed as one of these escapes is considered
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305as a character without special meaning by the regex engine, and will match
306"as is".
307
308Mnemonic: heI<x>adecimal.
309
9f5650a8 310=head4 Examples (assuming an ASCII platform)
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311
312 $str = "Perl";
313 $str =~ /\x50/; # Match, "\x50" is "P".
f822d0dd 314 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
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315 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
316
317 /\x{2603}\x{2602}/ # Snowman with an umbrella.
318 # The Unicode character 2603 is a snowman,
319 # the Unicode character 2602 is an umbrella.
320 /\x{263B}/ # Black smiling face.
321 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
322
323=head2 Modifiers
324
325A number of backslash sequences have to do with changing the character,
326or characters following them. C<\l> will lowercase the character following
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327it, while C<\u> will uppercase (or, more accurately, titlecase) the
328character following it. (They perform similar functionality as the
329functions C<lcfirst> and C<ucfirst>).
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330
331To uppercase or lowercase several characters, one might want to use
332C<\L> or C<\U>, which will lowercase/uppercase all characters following
e2cb52ee 333them, until either the end of the pattern, or the next occurrence of
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334C<\E>, whatever comes first. They perform similar functionality as the
335functions C<lc> and C<uc> do.
336
337C<\Q> is used to escape all characters following, up to the next C<\E>
338or the end of the pattern. C<\Q> adds a backslash to any character that
339isn't a letter, digit or underscore. This will ensure that any character
340between C<\Q> and C<\E> is matched literally, and will not be interpreted
341by the regexp engine.
342
343Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
344
345=head4 Examples
346
347 $sid = "sid";
348 $greg = "GrEg";
349 $miranda = "(Miranda)";
350 $str =~ /\u$sid/; # Matches 'Sid'
351 $str =~ /\L$greg/; # Matches 'greg'
352 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
353 # had been written as /\(Miranda\)/
354
355=head2 Character classes
356
357Perl regular expressions have a large range of character classes. Some of
358the character classes are written as a backslash sequence. We will briefly
359discuss those here; full details of character classes can be found in
360L<perlrecharclass>.
361
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362C<\w> is a character class that matches any single I<word> character
363(letters, digits, Unicode marks, and connector punctuation (like the
364underscore)). C<\d> is a character class that matches any decimal
365digit, while the character class C<\s> matches any whitespace character.
99d59c4d 366New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
418e7b04 367and vertical whitespace characters.
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368
369The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
370depending on various pragma and regular expression modifiers. See
371L<perlre>.
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372
373The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
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374character classes that match, respectively, any character that isn't a
375word character, digit, whitespace, horizontal whitespace, or vertical
376whitespace.
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377
378Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
379
380=head3 Unicode classes
381
382C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
383match a character that matches the given Unicode property; properties
384include things like "letter", or "thai character". Capitalizing the
385sequence to C<\PP> and C<\P{Property}> make the sequence match a character
386that doesn't match the given Unicode property. For more details, see
4948b50f 387L<perlrecharclass/Backslash sequences> and
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388L<perlunicode/Unicode Character Properties>.
389
390Mnemonic: I<p>roperty.
391
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392=head2 Referencing
393
394If capturing parenthesis are used in a regular expression, we can refer
395to the part of the source string that was matched, and match exactly the
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396same thing. There are three ways of referring to such I<backreference>:
397absolutely, relatively, and by name.
398
399=for later add link to perlrecapture
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400
401=head3 Absolute referencing
402
c27a5cfe 403Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
d8b950dc 404is a positive (unsigned) decimal number of any length is an absolute reference
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405to a capturing group.
406
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407I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
408been matched by that set of parentheses. Thus C<\g1> refers to the first
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409capture group in the regex.
410
411The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
412which avoids ambiguity when building a regex by concatenating shorter
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413strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
414C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
415probably not what you intended.
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416
417In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
418least I<N> capturing groups, or else I<N> will be considered an octal escape
419(but something like C<\18> is the same as C<\0018>, that is the octal escape
420C<"\001"> followed by a literal digit C<"8">).
421
422Mnemonic: I<g>roup.
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423
424=head4 Examples
425
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426 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
427 /(\w+) \1/; # Same thing; written old-style
428 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
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429
430
431=head3 Relative referencing
432
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433C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
434be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
435C<\g{-I<N>}>.
8a118206 436
c27a5cfe 437The big advantage of this form is that it makes it much easier to write
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438patterns with references that can be interpolated in larger patterns,
439even if the larger pattern also contains capture groups.
440
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441=head4 Examples
442
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443 /(A) # Group 1
444 ( # Group 2
445 (B) # Group 3
446 \g{-1} # Refers to group 3 (B)
447 \g{-3} # Refers to group 1 (A)
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448 )
449 /x; # Matches "ABBA".
450
451 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
452 /$qr$qr/ # Matches 'ababcdcd'.
453
454=head3 Named referencing
455
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456C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
457named capture group, dispensing completely with having to think about capture
458buffer positions.
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459
460To be compatible with .Net regular expressions, C<\g{name}> may also be
461written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
462
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463To prevent any ambiguity, I<name> must not start with a digit nor contain a
464hyphen.
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465
466=head4 Examples
467
468 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
469 /(?<word>\w+) \k{word}/ # Same.
470 /(?<word>\w+) \k<word>/ # Same.
471 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
472 # Match a four letter palindrome (e.g. "ABBA")
473
474=head2 Assertions
475
ac036724 476Assertions are conditions that have to be true; they don't actually
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477match parts of the substring. There are six assertions that are written as
478backslash sequences.
479
480=over 4
481
482=item \A
483
484C<\A> only matches at the beginning of the string. If the C</m> modifier
1726f7e8 485isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
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486modifier is used, then C</^/> matches internal newlines, but the meaning
487of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
488of the string regardless whether the C</m> modifier is used.
489
490=item \z, \Z
491
492C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
1726f7e8 493used, then C</\Z/> is equivalent to C</$/>, that is, it matches at the
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494end of the string, or before the newline at the end of the string. If the
495C</m> modifier is used, then C</$/> matches at internal newlines, but the
496meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
497the end of the string (or just before a trailing newline) regardless whether
498the C</m> modifier is used.
499
500C<\z> is just like C<\Z>, except that it will not match before a trailing
501newline. C<\z> will only match at the end of the string - regardless of the
502modifiers used, and not before a newline.
503
504=item \G
505
506C<\G> is usually only used in combination with the C</g> modifier. If the
507C</g> modifier is used (and the match is done in scalar context), Perl will
508remember where in the source string the last match ended, and the next time,
509it will start the match from where it ended the previous time.
510
511C<\G> matches the point where the previous match ended, or the beginning
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512of the string if there was no previous match.
513
514=for later add link to perlremodifiers
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515
516Mnemonic: I<G>lobal.
517
518=item \b, \B
519
520C<\b> matches at any place between a word and a non-word character; C<\B>
521matches at any place between characters where C<\b> doesn't match. C<\b>
522and C<\B> assume there's a non-word character before the beginning and after
523the end of the source string; so C<\b> will match at the beginning (or end)
524of the source string if the source string begins (or ends) with a word
525character. Otherwise, C<\B> will match.
526
527Mnemonic: I<b>oundary.
528
529=back
530
531=head4 Examples
532
533 "cat" =~ /\Acat/; # Match.
534 "cat" =~ /cat\Z/; # Match.
535 "cat\n" =~ /cat\Z/; # Match.
536 "cat\n" =~ /cat\z/; # No match.
537
538 "cat" =~ /\bcat\b/; # Matches.
539 "cats" =~ /\bcat\b/; # No match.
540 "cat" =~ /\bcat\B/; # No match.
541 "cats" =~ /\bcat\B/; # Match.
542
543 while ("cat dog" =~ /(\w+)/g) {
544 print $1; # Prints 'catdog'
545 }
546 while ("cat dog" =~ /\G(\w+)/g) {
547 print $1; # Prints 'cat'
548 }
549
550=head2 Misc
551
552Here we document the backslash sequences that don't fall in one of the
553categories above. They are:
554
555=over 4
556
557=item \C
558
559C<\C> always matches a single octet, even if the source string is encoded
560in UTF-8 format, and the character to be matched is a multi-octet character.
561C<\C> was introduced in perl 5.6.
562
563Mnemonic: oI<C>tet.
564
565=item \K
566
99d59c4d 567This is new in perl 5.10.0. Anything that is matched left of C<\K> is
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568not included in C<$&> - and will not be replaced if the pattern is
569used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
570instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
571
572Mnemonic: I<K>eep.
573
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574=item \N
575
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576This is a new experimental feature in perl 5.12.0. It matches any character
577that is not a newline. It is a short-hand for writing C<[^\n]>, and is
578identical to the C<.> metasymbol, except under the C</s> flag, which changes
579the meaning of C<.>, but not C<\N>.
df225385 580
e526e8bb 581Note that C<\N{...}> can mean a
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582L<named or numbered character
583|/Named or numbered characters and character sequences>.
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584
585Mnemonic: Complement of I<\n>.
586
8a118206 587=item \R
6b46370c 588X<\R>
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589
590C<\R> matches a I<generic newline>, that is, anything that is considered
591a newline by Unicode. This includes all characters matched by C<\v>
418e7b04 592(vertical whitespace), and the multi character sequence C<"\x0D\x0A">
8a118206 593(carriage return followed by a line feed, aka the network newline, or
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594the newline used in Windows text files). C<\R> is equivalent to
595C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
596character, it cannot be put inside a bracketed character class; C</[\R]/> is an
597error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
8a118206 598
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599Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
600and more importantly because Unicode recommends such a regular expression
601metacharacter, and suggests C<\R> as the notation.
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602
603=item \X
6b46370c 604X<\X>
8a118206 605
0111a78f 606This matches a Unicode I<extended grapheme cluster>.
8a118206 607
10fdd326 608C<\X> matches quite well what normal (non-Unicode-programmer) usage
0111a78f 609would consider a single character. As an example, consider a G with some sort
c670e63a 610of diacritic mark, such as an arrow. There is no such single character in
df225385 611Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
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612UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
613were a single character.
10fdd326 614
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615Mnemonic: eI<X>tended Unicode character.
616
617=back
618
619=head4 Examples
620
621 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
622
f822d0dd 623 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
d8b950dc 624 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
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625
626 "\n" =~ /^\R$/; # Match, \n is a generic newline.
627 "\r" =~ /^\R$/; # Match, \r is a generic newline.
628 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
629
630 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
631
632=cut