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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
df225385 27following the backslash is an ASCII punctuation (non-word) character (that is,
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28anything that is not a letter, digit, or underscore), then the backslash just
29takes away any special meaning of the character following it.
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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
b6538e4f 35you have warnings turned on, Perl issues a warning if you use such a
6b46370c 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
b6538e4f 51There is one exception. If you use an alphanumeric character as the
8a118206 52delimiter of your pattern (which you probably shouldn't do for readability
b6538e4f 53reasons), you have to escape the delimiter if you want to match
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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.
628253b8 78 \F Foldcase till \E. Not in [].
f822d0dd 79 \g{}, \g1 Named, absolute or relative backreference. Not in []
df225385 80 \G Pos assertion. Not in [].
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81 \h Character class for horizontal whitespace.
82 \H Character class for non horizontal whitespace.
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83 \k{}, \k<>, \k'' Named backreference. Not in [].
84 \K Keep the stuff left of \K. Not in [].
85 \l Lowercase next character. Not in [].
86 \L Lowercase till \E. Not in [].
8a118206 87 \n (Logical) newline character.
b3b85878 88 \N Any character but newline. Experimental. Not in [].
fb121860 89 \N{} Named or numbered (Unicode) character or sequence.
f0a2b745 90 \o{} Octal escape sequence.
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91 \p{}, \pP Character with the given Unicode property.
92 \P{}, \PP Character without the given Unicode property.
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93 \Q Quote (disable) pattern metacharacters till \E. Not
94 in [].
8a118206 95 \r Return character.
df225385 96 \R Generic new line. Not in [].
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97 \s Character class for whitespace.
98 \S Character class for non whitespace.
8a118206 99 \t Tab character.
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100 \u Titlecase next character. Not in [].
101 \U Uppercase till \E. Not in [].
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102 \v Character class for vertical whitespace.
103 \V Character class for non vertical whitespace.
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104 \w Character class for word characters.
105 \W Character class for non-word characters.
106 \x{}, \x00 Hexadecimal escape sequence.
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107 \X Unicode "extended grapheme cluster". Not in [].
108 \z End of string. Not in [].
109 \Z End of string. Not in [].
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110
111=head2 Character Escapes
112
113=head3 Fixed characters
114
e2cb52ee 115A handful of characters have a dedicated I<character escape>. The following
58151fe4 116table shows them, along with their ASCII code points (in decimal and hex),
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117their ASCII name, the control escape on ASCII platforms and a short
118description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
8a118206 119
4948b50f 120 Seq. Code Point ASCII Cntrl Description.
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121 Dec Hex
122 \a 7 07 BEL \cG alarm or bell
123 \b 8 08 BS \cH backspace [1]
124 \e 27 1B ESC \c[ escape character
125 \f 12 0C FF \cL form feed
126 \n 10 0A LF \cJ line feed [2]
127 \r 13 0D CR \cM carriage return
128 \t 9 09 TAB \cI tab
129
130=over 4
131
132=item [1]
133
301ba1af 134C<\b> is the backspace character only inside a character class. Outside a
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135character class, C<\b> is a word/non-word boundary.
136
137=item [2]
138
b6538e4f 139C<\n> matches a logical newline. Perl converts between C<\n> and your
f6993e9e 140OS's native newline character when reading from or writing to text files.
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141
142=back
143
144=head4 Example
145
146 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
147
148=head3 Control characters
149
150C<\c> is used to denote a control character; the character following C<\c>
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151determines the value of the construct. For example the value of C<\cA> is
152C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
153The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
154list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
155L<perlebcdic/OPERATOR DIFFERENCES>.
156
157Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
158string) is not valid. The backslash must be followed by another character.
159That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
160
161To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
162C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
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163
164Mnemonic: I<c>ontrol character.
165
166=head4 Example
167
168 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
169
fb121860 170=head3 Named or numbered characters and character sequences
8a118206 171
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172Unicode characters have a Unicode name and numeric code point (ordinal)
173value. Use the
e526e8bb 174C<\N{}> construct to specify a character by either of these values.
fb121860 175Certain sequences of characters also have names.
e526e8bb 176
fb121860 177To specify by name, the name of the character or character sequence goes
fbb93542 178between the curly braces.
e526e8bb 179
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180To specify a character by Unicode code point, use the form C<\N{U+I<code
181point>}>, where I<code point> is a number in hexadecimal that gives the
17148a1a 182code point that Unicode has assigned to the desired character. It is
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183customary but not required to use leading zeros to pad the number to 4
184digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
185rarely see it written without the two leading zeros. C<\N{U+0041}> means
186"A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
e526e8bb 187
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188It is even possible to give your own names to characters and character
189sequences. For details, see L<charnames>.
8a118206 190
8c37f1d0 191(There is an expanded internal form that you may see in debug output:
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192C<\N{U+I<code point>.I<code point>...}>.
193The C<...> means any number of these I<code point>s separated by dots.
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194This represents the sequence formed by the characters. This is an internal
195form only, subject to change, and you should not try to use it yourself.)
196
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197Mnemonic: I<N>amed character.
198
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199Note that a character or character sequence expressed as a named
200or numbered character is considered a character without special
fb121860 201meaning by the regex engine, and will match "as is".
df225385 202
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203=head4 Example
204
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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
f0a2b745 212There are two forms of octal escapes. Each is used to specify a character by
17148a1a 213its code point specified in octal notation.
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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
9645299c 224zero, but that makes \077 the largest code point specifiable.
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225
226In some contexts, a backslash followed by two or even one octal digits may be
227interpreted as an octal escape, sometimes with a warning, and because of some
228bugs, sometimes with surprising results. Also, if you are creating a regex
c69ca1d4 229out of smaller snippets concatenated together, and you use fewer than three
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230digits, the beginning of one snippet may be interpreted as adding digits to the
231ending of the snippet before it. See L</Absolute referencing> for more
232discussion and examples of the snippet problem.
8a118206 233
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234Note that a character expressed as an octal escape is considered
235a character without special meaning by the regex engine, and will match
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236"as is".
237
f0a2b745 238To summarize, the C<\o{}> form is always safe to use, and the other form is
17148a1a 239safe to use for code points through \077 when you use exactly three digits to
f0a2b745 240specify them.
8a118206 241
f0a2b745 242Mnemonic: I<0>ctal or I<o>ctal.
8a118206 243
f0a2b745 244=head4 Examples (assuming an ASCII platform)
8a118206 245
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246 $str = "Perl";
247 $str =~ /\o{120}/; # Match, "\120" is "P".
248 $str =~ /\120/; # Same.
249 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
250 $str =~ /\120+/; # Same.
251 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
252 /\o{23073}/ # Black foreground, white background smiling face.
253 /\o{4801234567}/ # Raises a warning, and yields chr(4)
254
255=head4 Disambiguation rules between old-style octal escapes and backreferences
256
257Octal escapes of the C<\000> form outside of bracketed character classes
258potentially clash with old-style backreferences. (see L</Absolute referencing>
259below). They both consist of a backslash followed by numbers. So Perl has to
260use heuristics to determine whether it is a backreference or an octal escape.
261Perl uses the following rules to disambiguate:
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262
263=over 4
264
265=item 1
266
353c6505 267If the backslash is followed by a single digit, it's a backreference.
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268
269=item 2
270
271If the first digit following the backslash is a 0, it's an octal escape.
272
273=item 3
274
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275If the number following the backslash is N (in decimal), and Perl already
276has seen N capture groups, Perl considers this a backreference. Otherwise,
277it considers it an octal escape. If N has more than three digits, Perl
278takes only the first three for the octal escape; the rest are matched as is.
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279
280 my $pat = "(" x 999;
281 $pat .= "a";
282 $pat .= ")" x 999;
283 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
284 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
f0a2b745 285 # and \1000 is seen as \100 (a '@') and a '0'
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286
287=back
288
17148a1a 289You can force a backreference interpretation always by using the C<\g{...}>
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290form. You can the force an octal interpretation always by using the C<\o{...}>
291form, or for numbers up through \077 (= 63 decimal), by using three digits,
292beginning with a "0".
293
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294=head3 Hexadecimal escapes
295
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296Like octal escapes, there are two forms of hexadecimal escapes, but both start
297with the same thing, C<\x>. This is followed by either exactly two hexadecimal
298digits forming a number, or a hexadecimal number of arbitrary length surrounded
299by curly braces. The hexadecimal number is the code point of the character you
300want to express.
8a118206 301
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302Note that a character expressed as one of these escapes is considered a
303character without special meaning by the regex engine, and will match
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304"as is".
305
306Mnemonic: heI<x>adecimal.
307
9f5650a8 308=head4 Examples (assuming an ASCII platform)
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309
310 $str = "Perl";
311 $str =~ /\x50/; # Match, "\x50" is "P".
f822d0dd 312 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
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313 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
314
315 /\x{2603}\x{2602}/ # Snowman with an umbrella.
316 # The Unicode character 2603 is a snowman,
317 # the Unicode character 2602 is an umbrella.
318 /\x{263B}/ # Black smiling face.
319 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
320
321=head2 Modifiers
322
323A number of backslash sequences have to do with changing the character,
324or characters following them. C<\l> will lowercase the character following
5f2b17ca 325it, while C<\u> will uppercase (or, more accurately, titlecase) the
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326character following it. They provide functionality similar to the
327functions C<lcfirst> and C<ucfirst>.
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328
329To uppercase or lowercase several characters, one might want to use
330C<\L> or C<\U>, which will lowercase/uppercase all characters following
b6538e4f 331them, until either the end of the pattern or the next occurrence of
17148a1a 332C<\E>, whichever comes first. They provide functionality similar to what
b6538e4f 333the functions C<lc> and C<uc> provide.
8a118206 334
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335C<\Q> is used to quote (disable) pattern metacharacters, up to the next
336C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
337that could have special meaning to Perl. In the ASCII range, it quotes
338every character that isn't a letter, digit, or underscore. See
339L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
340code points. Using this ensures that any character between C<\Q> and
341C<\E> will be matched literally, not interpreted as a metacharacter by
342the regex engine.
8a118206 343
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344C<\F> can be used to casefold all characters following, up to the next C<\E>
345or the end of the pattern. It provides the functionality similar to
346the C<fc> function.
347
348Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
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349
350=head4 Examples
351
352 $sid = "sid";
353 $greg = "GrEg";
354 $miranda = "(Miranda)";
355 $str =~ /\u$sid/; # Matches 'Sid'
356 $str =~ /\L$greg/; # Matches 'greg'
357 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
358 # had been written as /\(Miranda\)/
359
360=head2 Character classes
361
362Perl regular expressions have a large range of character classes. Some of
363the character classes are written as a backslash sequence. We will briefly
364discuss those here; full details of character classes can be found in
365L<perlrecharclass>.
366
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367C<\w> is a character class that matches any single I<word> character
368(letters, digits, Unicode marks, and connector punctuation (like the
369underscore)). C<\d> is a character class that matches any decimal
370digit, while the character class C<\s> matches any whitespace character.
99d59c4d 371New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
418e7b04 372and vertical whitespace characters.
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373
374The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
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375depending on various pragma and regular expression modifiers. It is
376possible to restrict the match to the ASCII range by using the C</a>
377regular expression modifier. See L<perlrecharclass>.
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378
379The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
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380character classes that match, respectively, any character that isn't a
381word character, digit, whitespace, horizontal whitespace, or vertical
382whitespace.
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383
384Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
385
386=head3 Unicode classes
387
388C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
389match a character that matches the given Unicode property; properties
390include things like "letter", or "thai character". Capitalizing the
391sequence to C<\PP> and C<\P{Property}> make the sequence match a character
392that doesn't match the given Unicode property. For more details, see
4948b50f 393L<perlrecharclass/Backslash sequences> and
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394L<perlunicode/Unicode Character Properties>.
395
396Mnemonic: I<p>roperty.
397
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398=head2 Referencing
399
400If capturing parenthesis are used in a regular expression, we can refer
401to the part of the source string that was matched, and match exactly the
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402same thing. There are three ways of referring to such I<backreference>:
403absolutely, relatively, and by name.
404
405=for later add link to perlrecapture
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406
407=head3 Absolute referencing
408
c27a5cfe 409Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
d8b950dc 410is a positive (unsigned) decimal number of any length is an absolute reference
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411to a capturing group.
412
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413I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
414been matched by that set of parentheses. Thus C<\g1> refers to the first
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415capture group in the regex.
416
417The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
418which avoids ambiguity when building a regex by concatenating shorter
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419strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
420C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
421probably not what you intended.
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422
423In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
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424least I<N> capturing groups, or else I<N> is considered an octal escape
425(but something like C<\18> is the same as C<\0018>; that is, the octal escape
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426C<"\001"> followed by a literal digit C<"8">).
427
428Mnemonic: I<g>roup.
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429
430=head4 Examples
431
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432 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
433 /(\w+) \1/; # Same thing; written old-style
434 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
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435
436
437=head3 Relative referencing
438
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439C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
440be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
441C<\g{-I<N>}>.
8a118206 442
c27a5cfe 443The big advantage of this form is that it makes it much easier to write
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444patterns with references that can be interpolated in larger patterns,
445even if the larger pattern also contains capture groups.
446
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447=head4 Examples
448
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449 /(A) # Group 1
450 ( # Group 2
451 (B) # Group 3
452 \g{-1} # Refers to group 3 (B)
453 \g{-3} # Refers to group 1 (A)
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454 )
455 /x; # Matches "ABBA".
456
457 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
458 /$qr$qr/ # Matches 'ababcdcd'.
459
460=head3 Named referencing
461
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462C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
463named capture group, dispensing completely with having to think about capture
464buffer positions.
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465
466To be compatible with .Net regular expressions, C<\g{name}> may also be
467written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
468
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469To prevent any ambiguity, I<name> must not start with a digit nor contain a
470hyphen.
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471
472=head4 Examples
473
474 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
475 /(?<word>\w+) \k{word}/ # Same.
476 /(?<word>\w+) \k<word>/ # Same.
477 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
478 # Match a four letter palindrome (e.g. "ABBA")
479
480=head2 Assertions
481
ac036724 482Assertions are conditions that have to be true; they don't actually
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483match parts of the substring. There are six assertions that are written as
484backslash sequences.
485
486=over 4
487
488=item \A
489
490C<\A> only matches at the beginning of the string. If the C</m> modifier
1726f7e8 491isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
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492modifier is used, then C</^/> matches internal newlines, but the meaning
493of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
494of the string regardless whether the C</m> modifier is used.
495
496=item \z, \Z
497
498C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
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499used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
500end of the string, or one before the newline at the end of the string. If the
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501C</m> modifier is used, then C</$/> matches at internal newlines, but the
502meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
503the end of the string (or just before a trailing newline) regardless whether
504the C</m> modifier is used.
505
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506C<\z> is just like C<\Z>, except that it does not match before a trailing
507newline. C<\z> matches at the end of the string only, regardless of the
508modifiers used, and not just before a newline. It is how to anchor the
509match to the true end of the string under all conditions.
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510
511=item \G
512
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513C<\G> is usually used only in combination with the C</g> modifier. If the
514C</g> modifier is used and the match is done in scalar context, Perl
515remembers where in the source string the last match ended, and the next time,
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516it will start the match from where it ended the previous time.
517
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518C<\G> matches the point where the previous match on that string ended,
519or the beginning of that string if there was no previous match.
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520
521=for later add link to perlremodifiers
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522
523Mnemonic: I<G>lobal.
524
525=item \b, \B
526
527C<\b> matches at any place between a word and a non-word character; C<\B>
528matches at any place between characters where C<\b> doesn't match. C<\b>
529and C<\B> assume there's a non-word character before the beginning and after
530the end of the source string; so C<\b> will match at the beginning (or end)
531of the source string if the source string begins (or ends) with a word
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532character. Otherwise, C<\B> will match.
533
534Do not use something like C<\b=head\d\b> and expect it to match the
535beginning of a line. It can't, because for there to be a boundary before
536the non-word "=", there must be a word character immediately previous.
537All boundary determinations look for word characters alone, not for
538non-words characters nor for string ends. It may help to understand how
539<\b> and <\B> work by equating them as follows:
540
541 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
542 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
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543
544Mnemonic: I<b>oundary.
545
546=back
547
548=head4 Examples
549
550 "cat" =~ /\Acat/; # Match.
551 "cat" =~ /cat\Z/; # Match.
552 "cat\n" =~ /cat\Z/; # Match.
553 "cat\n" =~ /cat\z/; # No match.
554
555 "cat" =~ /\bcat\b/; # Matches.
556 "cats" =~ /\bcat\b/; # No match.
557 "cat" =~ /\bcat\B/; # No match.
558 "cats" =~ /\bcat\B/; # Match.
559
560 while ("cat dog" =~ /(\w+)/g) {
561 print $1; # Prints 'catdog'
562 }
563 while ("cat dog" =~ /\G(\w+)/g) {
564 print $1; # Prints 'cat'
565 }
566
567=head2 Misc
568
569Here we document the backslash sequences that don't fall in one of the
b6538e4f 570categories above. These are:
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571
572=over 4
573
574=item \C
575
576C<\C> always matches a single octet, even if the source string is encoded
577in UTF-8 format, and the character to be matched is a multi-octet character.
69a6e56c 578This is very dangerous, because it violates
b6538e4f 579the logical character abstraction and can cause UTF-8 sequences to become malformed.
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580
581Mnemonic: oI<C>tet.
582
583=item \K
584
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585This appeared in perl 5.10.0. Anything matched left of C<\K> is
586not included in C<$&>, and will not be replaced if the pattern is
587used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
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588instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
589
590Mnemonic: I<K>eep.
591
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592=item \N
593
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594This is an experimental feature new to perl 5.12.0. It matches any character
595that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
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596identical to the C<.> metasymbol, except under the C</s> flag, which changes
597the meaning of C<.>, but not C<\N>.
df225385 598
e526e8bb 599Note that C<\N{...}> can mean a
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600L<named or numbered character
601|/Named or numbered characters and character sequences>.
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602
603Mnemonic: Complement of I<\n>.
604
8a118206 605=item \R
6b46370c 606X<\R>
8a118206 607
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608C<\R> matches a I<generic newline>; that is, anything considered a
609linebreak sequence by Unicode. This includes all characters matched by
610C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
611(carriage return followed by a line feed, sometimes called the network
612newline; it's the end of line sequence used in Microsoft text files opened
1978b668 613in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
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614reason it doesn't backtrack is that the sequence is considered
615inseparable. That means that
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616
617 "\x0D\x0A" =~ /^\R\x0A$/ # No match
618
619fails, because the C<\R> matches the entire string, and won't backtrack
620to match just the C<"\x0D">.) Since
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621C<\R> can match a sequence of more than one character, it cannot be put
622inside a bracketed character class; C</[\R]/> is an error; use C<\v>
623instead. C<\R> was introduced in perl 5.10.0.
8a118206 624
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625Note that this does not respect any locale that might be in effect; it
626matches according to the platform's native character set.
627
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628Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
629and more importantly because Unicode recommends such a regular expression
b6538e4f 630metacharacter, and suggests C<\R> as its notation.
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631
632=item \X
6b46370c 633X<\X>
8a118206 634
0111a78f 635This matches a Unicode I<extended grapheme cluster>.
8a118206 636
10fdd326 637C<\X> matches quite well what normal (non-Unicode-programmer) usage
0111a78f 638would consider a single character. As an example, consider a G with some sort
c670e63a 639of diacritic mark, such as an arrow. There is no such single character in
df225385 640Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
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641UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
642were a single character.
10fdd326 643
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644Mnemonic: eI<X>tended Unicode character.
645
646=back
647
648=head4 Examples
649
b6538e4f 650 "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
8a118206 651
f822d0dd 652 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
d8b950dc 653 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
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654
655 "\n" =~ /^\R$/; # Match, \n is a generic newline.
656 "\r" =~ /^\R$/; # Match, \r is a generic newline.
657 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
658
b6538e4f 659 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
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660
661=cut