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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
19
20=head2 The backslash
21
22In a regular expression, the backslash can perform one of two tasks:
23it either takes away the special meaning of the character following it
24(for instance, C<\|> matches a vertical bar, it's not an alternation),
25or it is the start of a backslash or escape sequence.
26
27The rules determining what it is are quite simple: if the character
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28following the backslash is an ASCII punctuation (non-word) character (that is,
29anything that is not a letter, digit or underscore), then the backslash just
30takes away the special meaning (if any) of the character following it.
31
32If the character following the backslash is an ASCII letter or an ASCII digit,
33then the sequence may be special; if so, it's listed below. A few letters have
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34not been used yet, so escaping them with a backslash doesn't change them to be
35special. A future version of Perl may assign a special meaning to them, so if
36you have warnings turned on, Perl will issue a warning if you use such a
37sequence. [1].
8a118206 38
e2cb52ee 39It is however guaranteed that backslash or escape sequences never have a
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40punctuation character following the backslash, not now, and not in a future
41version of Perl 5. So it is safe to put a backslash in front of a non-word
42character.
43
44Note that the backslash itself is special; if you want to match a backslash,
45you have to escape the backslash with a backslash: C</\\/> matches a single
46backslash.
47
48=over 4
49
50=item [1]
51
52There is one exception. If you use an alphanumerical character as the
53delimiter of your pattern (which you probably shouldn't do for readability
54reasons), you will have to escape the delimiter if you want to match
55it. Perl won't warn then. See also L<perlop/Gory details of parsing
56quoted constructs>.
57
58=back
59
60
61=head2 All the sequences and escapes
62
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63Those not usable within a bracketed character class (like C<[\da-z]>) are marked
64as C<Not in [].>
65
8a118206 66 \000 Octal escape sequence.
df225385 67 \1 Absolute backreference. Not in [].
8a118206 68 \a Alarm or bell.
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69 \A Beginning of string. Not in [].
70 \b Word/non-word boundary. (Backspace in []).
71 \B Not a word/non-word boundary. Not in [].
8a118206 72 \cX Control-X (X can be any ASCII character).
df225385 73 \C Single octet, even under UTF-8. Not in [].
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74 \d Character class for digits.
75 \D Character class for non-digits.
76 \e Escape character.
df225385 77 \E Turn off \Q, \L and \U processing. Not in [].
8a118206 78 \f Form feed.
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79 \g{}, \g1 Named, absolute or relative backreference. Not in [].
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 [].
e526e8bb 89 \N{} Named or numbered (Unicode) character.
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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
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114table shows them, along with their ASCII code points (in decimal and hex),
115their ASCII name, the control escape (see below) and a short description.
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116
117 Seq. Code Point ASCII Cntr Description.
118 Dec Hex
119 \a 7 07 BEL \cG alarm or bell
120 \b 8 08 BS \cH backspace [1]
121 \e 27 1B ESC \c[ escape character
122 \f 12 0C FF \cL form feed
123 \n 10 0A LF \cJ line feed [2]
124 \r 13 0D CR \cM carriage return
125 \t 9 09 TAB \cI tab
126
127=over 4
128
129=item [1]
130
131C<\b> is only the backspace character inside a character class. Outside a
132character class, C<\b> is a word/non-word boundary.
133
134=item [2]
135
136C<\n> matches a logical newline. Perl will convert between C<\n> and your
137OSses native newline character when reading from or writing to text files.
138
139=back
140
141=head4 Example
142
143 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
144
145=head3 Control characters
146
147C<\c> is used to denote a control character; the character following C<\c>
148is the name of the control character. For instance, C</\cM/> matches the
149character I<control-M> (a carriage return, code point 13). The case of the
150character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same
151character.
152
153Mnemonic: I<c>ontrol character.
154
155=head4 Example
156
157 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
158
e526e8bb 159=head3 Named or numbered characters
8a118206 160
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161All Unicode characters have a Unicode name and numeric ordinal value. Use the
162C<\N{}> construct to specify a character by either of these values.
163
164To specify by name, the name of the character goes between the curly braces.
165In this case, you have to C<use charnames> to load the Unicode names of the
166characters, otherwise Perl will complain.
167
168To specify by Unicode ordinal number, use the form
169C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
170hexadecimal that gives the ordinal number that Unicode has assigned to the
171desired character. It is customary (but not required) to use leading zeros to
172pad the number to 4 digits. Thus C<\N{U+0041}> means
173C<Latin Capital Letter A>, and you will rarely see it written without the two
174leading zeros. C<\N{U+0041}> means C<A> even on EBCDIC machines (where the
175ordinal value of C<A> is not 0x41).
176
177It is even possible to give your own names to characters, and even to short
178sequences of characters. For details, see L<charnames>.
8a118206 179
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180(There is an expanded internal form that you may see in debug output:
181C<\N{U+I<wide hex character>.I<wide hex character>...}>.
182The C<...> means any number of these I<wide hex character>s separated by dots.
183This represents the sequence formed by the characters. This is an internal
184form only, subject to change, and you should not try to use it yourself.)
185
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186Mnemonic: I<N>amed character.
187
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188Note that a character that is expressed as a named or numbered character is
189considered as a character without special meaning by the regex engine, and will
190match "as is".
df225385 191
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192=head4 Example
193
194 use charnames ':full'; # Loads the Unicode names.
195 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
196
197 use charnames 'Cyrillic'; # Loads Cyrillic names.
198 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
199
200=head3 Octal escapes
201
202Octal escapes consist of a backslash followed by two or three octal digits
203matching the code point of the character you want to use. This allows for
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204512 characters (C<\00> up to C<\777>) that can be expressed this way (but
205anything above C<\377> is deprecated).
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206Enough in pre-Unicode days, but most Unicode characters cannot be escaped
207this way.
208
209Note that a character that is expressed as an octal escape is considered
210as a character without special meaning by the regex engine, and will match
211"as is".
212
58151fe4 213=head4 Examples (assuming an ASCII platform)
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214
215 $str = "Perl";
216 $str =~ /\120/; # Match, "\120" is "P".
217 $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
218 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
219
220=head4 Caveat
221
222Octal escapes potentially clash with backreferences. They both consist
223of a backslash followed by numbers. So Perl has to use heuristics to
224determine whether it is a backreference or an octal escape. Perl uses
225the following rules:
226
227=over 4
228
229=item 1
230
353c6505 231If the backslash is followed by a single digit, it's a backreference.
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232
233=item 2
234
235If the first digit following the backslash is a 0, it's an octal escape.
236
237=item 3
238
239If the number following the backslash is N (decimal), and Perl already has
240seen N capture groups, Perl will consider this to be a backreference.
241Otherwise, it will consider it to be an octal escape. Note that if N > 999,
242Perl only takes the first three digits for the octal escape; the rest is
243matched as is.
244
245 my $pat = "(" x 999;
246 $pat .= "a";
247 $pat .= ")" x 999;
248 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
249 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
250 # and \1000 is seen as \100 (a '@') and a '0'.
251
252=back
253
254=head3 Hexadecimal escapes
255
58151fe4 256Hexadecimal escapes start with C<\x> and are then either followed by a
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257two digit hexadecimal number, or a hexadecimal number of arbitrary length
258surrounded by curly braces. The hexadecimal number is the code point of
259the character you want to express.
260
261Note that a character that is expressed as a hexadecimal escape is considered
262as a character without special meaning by the regex engine, and will match
263"as is".
264
265Mnemonic: heI<x>adecimal.
266
9f5650a8 267=head4 Examples (assuming an ASCII platform)
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268
269 $str = "Perl";
270 $str =~ /\x50/; # Match, "\x50" is "P".
271 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
272 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
273
274 /\x{2603}\x{2602}/ # Snowman with an umbrella.
275 # The Unicode character 2603 is a snowman,
276 # the Unicode character 2602 is an umbrella.
277 /\x{263B}/ # Black smiling face.
278 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
279
280=head2 Modifiers
281
282A number of backslash sequences have to do with changing the character,
283or characters following them. C<\l> will lowercase the character following
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284it, while C<\u> will uppercase (or, more accurately, titlecase) the
285character following it. (They perform similar functionality as the
286functions C<lcfirst> and C<ucfirst>).
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287
288To uppercase or lowercase several characters, one might want to use
289C<\L> or C<\U>, which will lowercase/uppercase all characters following
e2cb52ee 290them, until either the end of the pattern, or the next occurrence of
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291C<\E>, whatever comes first. They perform similar functionality as the
292functions C<lc> and C<uc> do.
293
294C<\Q> is used to escape all characters following, up to the next C<\E>
295or the end of the pattern. C<\Q> adds a backslash to any character that
296isn't a letter, digit or underscore. This will ensure that any character
297between C<\Q> and C<\E> is matched literally, and will not be interpreted
298by the regexp engine.
299
300Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
301
302=head4 Examples
303
304 $sid = "sid";
305 $greg = "GrEg";
306 $miranda = "(Miranda)";
307 $str =~ /\u$sid/; # Matches 'Sid'
308 $str =~ /\L$greg/; # Matches 'greg'
309 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
310 # had been written as /\(Miranda\)/
311
312=head2 Character classes
313
314Perl regular expressions have a large range of character classes. Some of
315the character classes are written as a backslash sequence. We will briefly
316discuss those here; full details of character classes can be found in
317L<perlrecharclass>.
318
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319C<\w> is a character class that matches any single I<word> character (letters,
320digits, underscore). C<\d> is a character class that matches any decimal digit,
418e7b04 321while the character class C<\s> matches any whitespace character.
99d59c4d 322New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
418e7b04 323and vertical whitespace characters.
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324
325The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
326character classes that match any character that isn't a word character,
418e7b04 327digit, whitespace, horizontal whitespace nor vertical whitespace.
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328
329Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
330
331=head3 Unicode classes
332
333C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
334match a character that matches the given Unicode property; properties
335include things like "letter", or "thai character". Capitalizing the
336sequence to C<\PP> and C<\P{Property}> make the sequence match a character
337that doesn't match the given Unicode property. For more details, see
338L<perlrecharclass/Backslashed sequences> and
339L<perlunicode/Unicode Character Properties>.
340
341Mnemonic: I<p>roperty.
342
343
344=head2 Referencing
345
346If capturing parenthesis are used in a regular expression, we can refer
347to the part of the source string that was matched, and match exactly the
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348same thing. There are three ways of referring to such I<backreference>:
349absolutely, relatively, and by name.
350
351=for later add link to perlrecapture
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352
353=head3 Absolute referencing
354
355A backslash sequence that starts with a backslash and is followed by a
356number is an absolute reference (but be aware of the caveat mentioned above).
df225385 357If the number is I<N>, it refers to the Nth set of parentheses - whatever
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358has been matched by that set of parenthesis has to be matched by the C<\N>
359as well.
360
361=head4 Examples
362
363 /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
364 /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
365
366
367=head3 Relative referencing
368
99d59c4d 369New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>.
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370C<\g> takes a number as argument, with the number in curly braces (the
371braces are optional). If the number (N) does not have a sign, it's a reference
372to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
373C<\g> always refers to a capture group and will never be seen as an octal
e2cb52ee 374escape). If the number is negative, the reference is relative, referring to
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375the Nth group before the C<\g{-N}>.
376
377The big advantage of C<\g{-N}> is that it makes it much easier to write
378patterns with references that can be interpolated in larger patterns,
379even if the larger pattern also contains capture groups.
380
381Mnemonic: I<g>roup.
382
383=head4 Examples
384
385 /(A) # Buffer 1
386 ( # Buffer 2
387 (B) # Buffer 3
388 \g{-1} # Refers to buffer 3 (B)
389 \g{-3} # Refers to buffer 1 (A)
390 )
391 /x; # Matches "ABBA".
392
393 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
394 /$qr$qr/ # Matches 'ababcdcd'.
395
396=head3 Named referencing
397
99d59c4d 398Also new in perl 5.10.0 is the use of named capture buffers, which can be
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399referred to by name. This is done with C<\g{name}>, which is a
400backreference to the capture buffer with the name I<name>.
401
402To be compatible with .Net regular expressions, C<\g{name}> may also be
403written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
404
405Note that C<\g{}> has the potential to be ambiguous, as it could be a named
406reference, or an absolute or relative reference (if its argument is numeric).
df225385 407However, names are not allowed to start with digits, nor are they allowed to
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408contain a hyphen, so there is no ambiguity.
409
410=head4 Examples
411
412 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
413 /(?<word>\w+) \k{word}/ # Same.
414 /(?<word>\w+) \k<word>/ # Same.
415 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
416 # Match a four letter palindrome (e.g. "ABBA")
417
418=head2 Assertions
419
ac036724 420Assertions are conditions that have to be true; they don't actually
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421match parts of the substring. There are six assertions that are written as
422backslash sequences.
423
424=over 4
425
426=item \A
427
428C<\A> only matches at the beginning of the string. If the C</m> modifier
429isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
430modifier is used, then C</^/> matches internal newlines, but the meaning
431of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
432of the string regardless whether the C</m> modifier is used.
433
434=item \z, \Z
435
436C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
437used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
438end of the string, or before the newline at the end of the string. If the
439C</m> modifier is used, then C</$/> matches at internal newlines, but the
440meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
441the end of the string (or just before a trailing newline) regardless whether
442the C</m> modifier is used.
443
444C<\z> is just like C<\Z>, except that it will not match before a trailing
445newline. C<\z> will only match at the end of the string - regardless of the
446modifiers used, and not before a newline.
447
448=item \G
449
450C<\G> is usually only used in combination with the C</g> modifier. If the
451C</g> modifier is used (and the match is done in scalar context), Perl will
452remember where in the source string the last match ended, and the next time,
453it will start the match from where it ended the previous time.
454
455C<\G> matches the point where the previous match ended, or the beginning
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456of the string if there was no previous match.
457
458=for later add link to perlremodifiers
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459
460Mnemonic: I<G>lobal.
461
462=item \b, \B
463
464C<\b> matches at any place between a word and a non-word character; C<\B>
465matches at any place between characters where C<\b> doesn't match. C<\b>
466and C<\B> assume there's a non-word character before the beginning and after
467the end of the source string; so C<\b> will match at the beginning (or end)
468of the source string if the source string begins (or ends) with a word
469character. Otherwise, C<\B> will match.
470
471Mnemonic: I<b>oundary.
472
473=back
474
475=head4 Examples
476
477 "cat" =~ /\Acat/; # Match.
478 "cat" =~ /cat\Z/; # Match.
479 "cat\n" =~ /cat\Z/; # Match.
480 "cat\n" =~ /cat\z/; # No match.
481
482 "cat" =~ /\bcat\b/; # Matches.
483 "cats" =~ /\bcat\b/; # No match.
484 "cat" =~ /\bcat\B/; # No match.
485 "cats" =~ /\bcat\B/; # Match.
486
487 while ("cat dog" =~ /(\w+)/g) {
488 print $1; # Prints 'catdog'
489 }
490 while ("cat dog" =~ /\G(\w+)/g) {
491 print $1; # Prints 'cat'
492 }
493
494=head2 Misc
495
496Here we document the backslash sequences that don't fall in one of the
497categories above. They are:
498
499=over 4
500
501=item \C
502
503C<\C> always matches a single octet, even if the source string is encoded
504in UTF-8 format, and the character to be matched is a multi-octet character.
505C<\C> was introduced in perl 5.6.
506
507Mnemonic: oI<C>tet.
508
509=item \K
510
99d59c4d 511This is new in perl 5.10.0. Anything that is matched left of C<\K> is
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512not included in C<$&> - and will not be replaced if the pattern is
513used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
514instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
515
516Mnemonic: I<K>eep.
517
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518=item \N
519
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520This is a new experimental feature in perl 5.12.0. It matches any character
521that is not a newline. It is a short-hand for writing C<[^\n]>, and is
522identical to the C<.> metasymbol, except under the C</s> flag, which changes
523the meaning of C<.>, but not C<\N>.
df225385 524
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525Note that C<\N{...}> can mean a
526L<named or numbered character|/Named or numbered characters>.
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527
528Mnemonic: Complement of I<\n>.
529
8a118206 530=item \R
6b46370c 531X<\R>
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532
533C<\R> matches a I<generic newline>, that is, anything that is considered
534a newline by Unicode. This includes all characters matched by C<\v>
418e7b04 535(vertical whitespace), and the multi character sequence C<"\x0D\x0A">
8a118206 536(carriage return followed by a line feed, aka the network newline, or
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537the newline used in Windows text files). C<\R> is equivalent to
538C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
539character, it cannot be put inside a bracketed character class; C</[\R]/> is an
540error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
8a118206 541
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542Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
543and more importantly because Unicode recommends such a regular expression
544metacharacter, and suggests C<\R> as the notation.
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545
546=item \X
6b46370c 547X<\X>
8a118206 548
0111a78f 549This matches a Unicode I<extended grapheme cluster>.
8a118206 550
10fdd326 551C<\X> matches quite well what normal (non-Unicode-programmer) usage
0111a78f 552would consider a single character. As an example, consider a G with some sort
c670e63a 553of diacritic mark, such as an arrow. There is no such single character in
df225385 554Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
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555UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
556were a single character.
10fdd326 557
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558Mnemonic: eI<X>tended Unicode character.
559
560=back
561
562=head4 Examples
563
564 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
565
566 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
567 $str =~ s/(.)\K\1//g; # Delete duplicated characters.
568
569 "\n" =~ /^\R$/; # Match, \n is a generic newline.
570 "\r" =~ /^\R$/; # Match, \r is a generic newline.
571 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
572
573 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
574
575=cut