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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.
df225385 68 \A Beginning of string. Not in [].
64935bc6 69 \b{}, \b Boundary. (\b is a backspace in []).
bc1d9728 70 \B{}, \B Not a boundary. Not in [].
f321be7e 71 \cX Control-X.
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72 \d Character class for digits.
73 \D Character class for non-digits.
74 \e Escape character.
df225385 75 \E Turn off \Q, \L and \U processing. Not in [].
8a118206 76 \f Form feed.
628253b8 77 \F Foldcase till \E. Not in [].
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78 \g{}, \g1 Named, absolute or relative backreference.
79 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.
4e5e0888 88 \N Any character but newline. 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> alone is a word-character/non-word-character
136boundary, and C<\b{}> is some other type of boundary.
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137
138=item [2]
139
b6538e4f 140C<\n> matches a logical newline. Perl converts between C<\n> and your
f6993e9e 141OS's native newline character when reading from or writing to text files.
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142
143=back
144
145=head4 Example
146
147 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
148
149=head3 Control characters
150
151C<\c> is used to denote a control character; the character following C<\c>
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152determines the value of the construct. For example the value of C<\cA> is
153C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
154The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
155list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
156L<perlebcdic/OPERATOR DIFFERENCES>.
157
158Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
159string) is not valid. The backslash must be followed by another character.
160That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
161
162To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
163C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
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164
165Mnemonic: I<c>ontrol character.
166
167=head4 Example
168
169 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
170
fb121860 171=head3 Named or numbered characters and character sequences
8a118206 172
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173Unicode characters have a Unicode name and numeric code point (ordinal)
174value. Use the
e526e8bb 175C<\N{}> construct to specify a character by either of these values.
fb121860 176Certain sequences of characters also have names.
e526e8bb 177
fb121860 178To specify by name, the name of the character or character sequence goes
fbb93542 179between the curly braces.
e526e8bb 180
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181To specify a character by Unicode code point, use the form C<\N{U+I<code
182point>}>, where I<code point> is a number in hexadecimal that gives the
17148a1a 183code point that Unicode has assigned to the desired character. It is
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184customary but not required to use leading zeros to pad the number to 4
185digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
186rarely see it written without the two leading zeros. C<\N{U+0041}> means
187"A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
e526e8bb 188
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189It is even possible to give your own names to characters and character
190sequences. For details, see L<charnames>.
8a118206 191
8c37f1d0 192(There is an expanded internal form that you may see in debug output:
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193C<\N{U+I<code point>.I<code point>...}>.
194The C<...> means any number of these I<code point>s separated by dots.
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195This represents the sequence formed by the characters. This is an internal
196form only, subject to change, and you should not try to use it yourself.)
197
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198Mnemonic: I<N>amed character.
199
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200Note that a character or character sequence expressed as a named
201or numbered character is considered a character without special
fb121860 202meaning by the regex engine, and will match "as is".
df225385 203
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204=head4 Example
205
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206 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
207
208 use charnames 'Cyrillic'; # Loads Cyrillic names.
209 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
210
211=head3 Octal escapes
212
f0a2b745 213There are two forms of octal escapes. Each is used to specify a character by
17148a1a 214its code point specified in octal notation.
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215
216One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
217represent one or more octal digits. It can be used for any Unicode character.
218
219It was introduced to avoid the potential problems with the other form,
220available in all Perls. That form consists of a backslash followed by three
221octal digits. One problem with this form is that it can look exactly like an
222old-style backreference (see
223L</Disambiguation rules between old-style octal escapes and backreferences>
224below.) You can avoid this by making the first of the three digits always a
9645299c 225zero, but that makes \077 the largest code point specifiable.
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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.
8a118206 234
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235Note that a character expressed as an octal escape is considered
236a character without special meaning by the regex engine, and will match
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237"as is".
238
f0a2b745 239To summarize, the C<\o{}> form is always safe to use, and the other form is
17148a1a 240safe to use for code points through \077 when you use exactly three digits to
f0a2b745 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.
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250 $str =~ /\o{120}+/; # Match, "\120" is "P",
251 # it's repeated at least once.
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252 $str =~ /\120+/; # Same.
253 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
254 /\o{23073}/ # Black foreground, white background smiling face.
f321be7e 255 /\o{4801234567}/ # Raises a warning, and yields chr(4).
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256
257=head4 Disambiguation rules between old-style octal escapes and backreferences
258
259Octal escapes of the C<\000> form outside of bracketed character classes
f321be7e 260potentially clash with old-style backreferences (see L</Absolute referencing>
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261below). They both consist of a backslash followed by numbers. So Perl has to
262use heuristics to determine whether it is a backreference or an octal escape.
263Perl uses the following rules to disambiguate:
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264
265=over 4
266
267=item 1
268
353c6505 269If the backslash is followed by a single digit, it's a backreference.
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270
271=item 2
272
273If the first digit following the backslash is a 0, it's an octal escape.
274
275=item 3
276
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277If the number following the backslash is N (in decimal), and Perl already
278has seen N capture groups, Perl considers this a backreference. Otherwise,
279it considers it an octal escape. If N has more than three digits, Perl
280takes only the first three for the octal escape; the 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
f321be7e 287 # and \1000 is seen as \100 (a '@') and a '0'.
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288
289=back
290
17148a1a 291You can force a backreference interpretation always by using the C<\g{...}>
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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
f0a2b745 298Like octal escapes, there are two forms of hexadecimal escapes, but both start
febd1aee 299with the sequence C<\x>. This is followed by either exactly two hexadecimal
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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
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304Note that a character expressed as one of these escapes is considered a
305character without special meaning by the regex engine, and will match
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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
5f2b17ca 327it, while C<\u> will uppercase (or, more accurately, titlecase) the
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328character following it. They provide functionality similar to 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
b6538e4f 333them, until either the end of the pattern or the next occurrence of
17148a1a 334C<\E>, whichever comes first. They provide functionality similar to what
b6538e4f 335the functions C<lc> and C<uc> provide.
8a118206 336
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337C<\Q> is used to quote (disable) pattern metacharacters, up to the next
338C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
339that could have special meaning to Perl. In the ASCII range, it quotes
340every character that isn't a letter, digit, or underscore. See
341L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
342code points. Using this ensures that any character between C<\Q> and
343C<\E> will be matched literally, not interpreted as a metacharacter by
344the regex engine.
8a118206 345
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346C<\F> can be used to casefold all characters following, up to the next C<\E>
347or the end of the pattern. It provides the functionality similar to
348the C<fc> function.
349
350Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
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351
352=head4 Examples
353
354 $sid = "sid";
355 $greg = "GrEg";
356 $miranda = "(Miranda)";
357 $str =~ /\u$sid/; # Matches 'Sid'
358 $str =~ /\L$greg/; # Matches 'greg'
359 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
360 # had been written as /\(Miranda\)/
361
362=head2 Character classes
363
364Perl regular expressions have a large range of character classes. Some of
365the character classes are written as a backslash sequence. We will briefly
366discuss those here; full details of character classes can be found in
367L<perlrecharclass>.
368
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369C<\w> is a character class that matches any single I<word> character
370(letters, digits, Unicode marks, and connector punctuation (like the
371underscore)). C<\d> is a character class that matches any decimal
372digit, while the character class C<\s> matches any whitespace character.
99d59c4d 373New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
418e7b04 374and vertical whitespace characters.
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375
376The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
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377depending on various pragma and regular expression modifiers. It is
378possible to restrict the match to the ASCII range by using the C</a>
379regular expression modifier. See L<perlrecharclass>.
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380
381The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
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382character classes that match, respectively, any character that isn't a
383word character, digit, whitespace, horizontal whitespace, or vertical
384whitespace.
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385
386Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
387
388=head3 Unicode classes
389
390C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
391match a character that matches the given Unicode property; properties
392include things like "letter", or "thai character". Capitalizing the
393sequence to C<\PP> and C<\P{Property}> make the sequence match a character
394that doesn't match the given Unicode property. For more details, see
4948b50f 395L<perlrecharclass/Backslash sequences> and
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396L<perlunicode/Unicode Character Properties>.
397
398Mnemonic: I<p>roperty.
399
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400=head2 Referencing
401
402If capturing parenthesis are used in a regular expression, we can refer
403to the part of the source string that was matched, and match exactly the
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404same thing. There are three ways of referring to such I<backreference>:
405absolutely, relatively, and by name.
406
407=for later add link to perlrecapture
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408
409=head3 Absolute referencing
410
c27a5cfe 411Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
d8b950dc 412is a positive (unsigned) decimal number of any length is an absolute reference
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413to a capturing group.
414
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415I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
416been matched by that set of parentheses. Thus C<\g1> refers to the first
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417capture group in the regex.
418
419The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
420which avoids ambiguity when building a regex by concatenating shorter
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421strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
422C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
423probably not what you intended.
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424
425In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
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426least I<N> capturing groups, or else I<N> is considered an octal escape
427(but something like C<\18> is the same as C<\0018>; that is, the octal escape
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428C<"\001"> followed by a literal digit C<"8">).
429
430Mnemonic: I<g>roup.
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431
432=head4 Examples
433
c27a5cfe 434 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
f321be7e 435 /(\w+) \1/; # Same thing; written old-style.
c27a5cfe 436 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
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437
438
439=head3 Relative referencing
440
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441C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
442be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
443C<\g{-I<N>}>.
8a118206 444
c27a5cfe 445The big advantage of this form is that it makes it much easier to write
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446patterns with references that can be interpolated in larger patterns,
447even if the larger pattern also contains capture groups.
448
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449=head4 Examples
450
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451 /(A) # Group 1
452 ( # Group 2
453 (B) # Group 3
454 \g{-1} # Refers to group 3 (B)
455 \g{-3} # Refers to group 1 (A)
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456 )
457 /x; # Matches "ABBA".
458
459 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
460 /$qr$qr/ # Matches 'ababcdcd'.
461
462=head3 Named referencing
463
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464C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
465named capture group, dispensing completely with having to think about capture
466buffer positions.
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467
468To be compatible with .Net regular expressions, C<\g{name}> may also be
469written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
470
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471To prevent any ambiguity, I<name> must not start with a digit nor contain a
472hyphen.
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473
474=head4 Examples
475
476 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
477 /(?<word>\w+) \k{word}/ # Same.
478 /(?<word>\w+) \k<word>/ # Same.
479 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
480 # Match a four letter palindrome (e.g. "ABBA")
481
482=head2 Assertions
483
ac036724 484Assertions are conditions that have to be true; they don't actually
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485match parts of the substring. There are six assertions that are written as
486backslash sequences.
487
488=over 4
489
490=item \A
491
492C<\A> only matches at the beginning of the string. If the C</m> modifier
1726f7e8 493isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
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494modifier is used, then C</^/> matches internal newlines, but the meaning
495of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
496of the string regardless whether the C</m> modifier is used.
497
498=item \z, \Z
499
500C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
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501used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
502end of the string, or one before the newline at the end of the string. If the
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503C</m> modifier is used, then C</$/> matches at internal newlines, but the
504meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
505the end of the string (or just before a trailing newline) regardless whether
506the C</m> modifier is used.
507
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508C<\z> is just like C<\Z>, except that it does not match before a trailing
509newline. C<\z> matches at the end of the string only, regardless of the
510modifiers used, and not just before a newline. It is how to anchor the
511match to the true end of the string under all conditions.
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512
513=item \G
514
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515C<\G> is usually used only in combination with the C</g> modifier. If the
516C</g> modifier is used and the match is done in scalar context, Perl
517remembers where in the source string the last match ended, and the next time,
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518it will start the match from where it ended the previous time.
519
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520C<\G> matches the point where the previous match on that string ended,
521or the beginning of that string if there was no previous match.
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522
523=for later add link to perlremodifiers
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524
525Mnemonic: I<G>lobal.
526
64935bc6 527=item \b{}, \b, \B{}, \B
8a118206 528
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529C<\b{...}>, available starting in v5.22, matches a boundary (between two
530characters, or before the first character of the string, or after the
531final character of the string) based on the Unicode rules for the
532boundary type specified inside the braces. The currently known boundary
533types are given a few paragraphs below. C<\B{...}> matches at any place
534between characters where C<\b{...}> of the same type doesn't match.
535
536C<\b> when not immediately followed by a C<"{"> matches at any place
537between a word (something matched by C<\w>) and a non-word character
538(C<\W>); C<\B> when not immediately followed by a C<"{"> matches at any
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539place between characters where C<\b> doesn't match. To get better
540word matching of natural language text, see L<\b{wb}> below.
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541
542C<\b>
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543and C<\B> assume there's a non-word character before the beginning and after
544the end of the source string; so C<\b> will match at the beginning (or end)
545of the source string if the source string begins (or ends) with a word
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546character. Otherwise, C<\B> will match.
547
548Do not use something like C<\b=head\d\b> and expect it to match the
549beginning of a line. It can't, because for there to be a boundary before
550the non-word "=", there must be a word character immediately previous.
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551All plain C<\b> and C<\B> boundary determinations look for word
552characters alone, not for
553non-word characters nor for string ends. It may help to understand how
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554<\b> and <\B> work by equating them as follows:
555
556 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
557 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
8a118206 558
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559In contrast, C<\b{...}> and C<\B{...}> may or may not match at the
560beginning and end of the line, depending on the boundary type. These
561implement the Unicode default boundaries, specified in
562L<http://www.unicode.org/reports/tr29/>.
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563The boundary types currently available are:
564
565=over
566
567=item C<\b{gcb}> or C<\b{g}>
568
569This matches a Unicode "Grapheme Cluster Boundary". (Actually Perl
570always uses the improved "extended" grapheme cluster"). These are
571explained below under L</C<\X>>. In fact, C<\X> is another way to get
572the same functionality. It is equivalent to C</.+?\b{gcb}/>. Use
573whichever is most convenient for your situation.
574
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575=item C<\b{sb}>
576
577This matches a Unicode "Sentence Boundary". This is an aid to parsing
578natural language sentences. It gives good, but imperfect results. For
579example, it thinks that "Mr. Smith" is two sentences. More details are
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580at L<http://www.unicode.org/reports/tr29/>. Note also that it thinks
581that anything matching L</\R> (except form feed and vertical tab) is a
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582sentence boundary. C<\b{sb}> works with text designed for
583word-processors which wrap lines
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584automatically for display, but hard-coded line boundaries are considered
585to be essentially the ends of text blocks (paragraphs really), and hence
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586the ends of sententces. C<\b{sb}> doesn't do well with text containing
587embedded newlines, like the source text of the document you are reading.
588Such text needs to be preprocessed to get rid of the line separators
589before looking for sentence boundaries. Some people view this as a bug
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590in the Unicode standard, and this behavior is quite subject to change in
591future Perl versions.
06ae2722 592
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593=item C<\b{wb}>
594
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595This matches a Unicode "Word Boundary", but tailored to Perl
596expectations. This gives better (though not
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597perfect) results for natural language processing than plain C<\b>
598(without braces) does. For example, it understands that apostrophes can
54bdcd8e 599be in the middle of words and that parentheses aren't (see the examples
39a8449e 600below). More details are at L<http://www.unicode.org/reports/tr29/>.
ae3bb8ea 601
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602The current Unicode definition of a Word Boundary matches between every
603white space character. Perl tailors this, starting in version 5.24, to
604generally not break up spans of white space, just as plain C<\b> has
605always functioned. This allows C<\b{wb}> to be a drop-in replacement for
606C<\b>, but with generally better results for natural language
607processing. (The exception to this tailoring is when a span of white
608space is immediately followed by something like U+0303, COMBINING TILDE.
609If the final space character in the span is a horizontal white space, it
610is broken out so that it attaches instead to the combining character.
611To be precise, if a span of white space that ends in a horizontal space
612has the character immediately following it have either of the Word
613Boundary property values "Extend" or "Format", the boundary between the
614final horizontal space character and the rest of the span matches
615C<\b{wb}>. In all other cases the boundary between two white space
616characters matches C<\B{wb}>.)
617
ae3bb8ea 618=back
64935bc6 619
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620It is important to realize when you use these Unicode boundaries,
621that you are taking a risk that a future version of Perl which contains
622a later version of the Unicode Standard will not work precisely the same
623way as it did when your code was written. These rules are not
624considered stable and have been somewhat more subject to change than the
625rest of the Standard. Unicode reserves the right to change them at
626will, and Perl reserves the right to update its implementation to
627Unicode's new rules. In the past, some changes have been because new
628characters have been added to the Standard which have different
629characteristics than all previous characters, so new rules are
630formulated for handling them. These should not cause any backward
631compatibility issues. But some changes have changed the treatment of
632existing characters because the Unicode Technical Committee has decided
633that the change is warranted for whatever reason. This could be to fix
634a bug, or because they think better results are obtained with the new
635rule.
636
637It is also important to realize that these are default boundary
638definitions, and that implementations may wish to tailor the results for
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639particular purposes and locales. For example, some languages, such as
640Japanese and Thai, require dictionary lookup to determine word
641boundaries.
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642
643Unicode defines a fourth boundary type, accessible through the
644L<Unicode::LineBreak> module.
645
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646Mnemonic: I<b>oundary.
647
648=back
649
650=head4 Examples
651
652 "cat" =~ /\Acat/; # Match.
653 "cat" =~ /cat\Z/; # Match.
654 "cat\n" =~ /cat\Z/; # Match.
655 "cat\n" =~ /cat\z/; # No match.
656
657 "cat" =~ /\bcat\b/; # Matches.
658 "cats" =~ /\bcat\b/; # No match.
659 "cat" =~ /\bcat\B/; # No match.
660 "cats" =~ /\bcat\B/; # Match.
661
662 while ("cat dog" =~ /(\w+)/g) {
663 print $1; # Prints 'catdog'
664 }
665 while ("cat dog" =~ /\G(\w+)/g) {
666 print $1; # Prints 'cat'
667 }
668
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669 my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
670 print join("|", $s =~ m/ ( .+? \b ) /xg), "\n";
671 print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
ae3bb8ea 672 prints
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673 He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
674 He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"
ae3bb8ea 675
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676=head2 Misc
677
678Here we document the backslash sequences that don't fall in one of the
b6538e4f 679categories above. These are:
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680
681=over 4
682
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683=item \K
684
b6538e4f
TC
685This appeared in perl 5.10.0. Anything matched left of C<\K> is
686not included in C<$&>, and will not be replaced if the pattern is
687used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
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688instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
689
690Mnemonic: I<K>eep.
691
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692=item \N
693
2171640d 694This feature, available starting in v5.12, matches any character
b6538e4f 695that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
b3b85878
KW
696identical to the C<.> metasymbol, except under the C</s> flag, which changes
697the meaning of C<.>, but not C<\N>.
df225385 698
e526e8bb 699Note that C<\N{...}> can mean a
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KW
700L<named or numbered character
701|/Named or numbered characters and character sequences>.
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702
703Mnemonic: Complement of I<\n>.
704
8a118206 705=item \R
6b46370c 706X<\R>
8a118206 707
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TC
708C<\R> matches a I<generic newline>; that is, anything considered a
709linebreak sequence by Unicode. This includes all characters matched by
710C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
711(carriage return followed by a line feed, sometimes called the network
712newline; it's the end of line sequence used in Microsoft text files opened
1978b668 713in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
040ac264
FC
714reason it doesn't backtrack is that the sequence is considered
715inseparable. That means that
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KW
716
717 "\x0D\x0A" =~ /^\R\x0A$/ # No match
718
719fails, because the C<\R> matches the entire string, and won't backtrack
720to match just the C<"\x0D">.) Since
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721C<\R> can match a sequence of more than one character, it cannot be put
722inside a bracketed character class; C</[\R]/> is an error; use C<\v>
723instead. C<\R> was introduced in perl 5.10.0.
8a118206 724
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725Note that this does not respect any locale that might be in effect; it
726matches according to the platform's native character set.
727
10fdd326
JH
728Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
729and more importantly because Unicode recommends such a regular expression
b6538e4f 730metacharacter, and suggests C<\R> as its notation.
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731
732=item \X
6b46370c 733X<\X>
8a118206 734
0111a78f 735This matches a Unicode I<extended grapheme cluster>.
8a118206 736
10fdd326 737C<\X> matches quite well what normal (non-Unicode-programmer) usage
0111a78f 738would consider a single character. As an example, consider a G with some sort
c670e63a 739of diacritic mark, such as an arrow. There is no such single character in
df225385 740Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
c670e63a
KW
741UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
742were a single character.
10fdd326 743
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744The match is greedy and non-backtracking, so that the cluster is never
745broken up into smaller components.
746
64935bc6
KW
747See also L<C<\b{gcb}>|/\b{}, \b, \B{}, \B>.
748
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749Mnemonic: eI<X>tended Unicode character.
750
751=back
752
753=head4 Examples
754
f822d0dd 755 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
d8b950dc 756 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
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757
758 "\n" =~ /^\R$/; # Match, \n is a generic newline.
759 "\r" =~ /^\R$/; # Match, \r is a generic newline.
760 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
761
b6538e4f 762 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
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763
764=cut