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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
28following the backslash is a punctuation (non-word) character (that is,
29anything that is not a letter, digit or underscore), then the backslash
30just takes away the special meaning (if any) of the character following
31it.
32
33If the character following the backslash is a letter or a digit, then the
34sequence may be special; if so, it's listed below. A few letters have not
35been used yet, and escaping them with a backslash is safe for now, but a
36future version of Perl may assign a special meaning to it. However, if you
37have warnings turned on, Perl will issue a warning if you use such a sequence.
38[1].
39
40It is however garanteed that backslash or escape sequences never have a
41punctuation character following the backslash, not now, and not in a future
42version of Perl 5. So it is safe to put a backslash in front of a non-word
43character.
44
45Note that the backslash itself is special; if you want to match a backslash,
46you have to escape the backslash with a backslash: C</\\/> matches a single
47backslash.
48
49=over 4
50
51=item [1]
52
53There is one exception. If you use an alphanumerical character as the
54delimiter of your pattern (which you probably shouldn't do for readability
55reasons), you will have to escape the delimiter if you want to match
56it. Perl won't warn then. See also L<perlop/Gory details of parsing
57quoted constructs>.
58
59=back
60
61
62=head2 All the sequences and escapes
63
64 \000 Octal escape sequence.
65 \1 Absolute backreference.
66 \a Alarm or bell.
67 \A Beginning of string.
68 \b Word/non-word boundary. (Backspace in a char class).
69 \B Not a word/non-word boundary.
70 \cX Control-X (X can be any ASCII character).
71 \C Single octet, even under UTF-8.
72 \d Character class for digits.
73 \D Character class for non-digits.
74 \e Escape character.
75 \E Turn off \Q, \L and \U processing.
76 \f Form feed.
77 \g{}, \g1 Named, absolute or relative backreference.
78 \G Pos assertion.
79 \h Character class for horizontal white space.
80 \H Character class for non horizontal white space.
81 \k{}, \k<>, \k'' Named backreference.
82 \K Keep the stuff left of \K.
83 \l Lowercase next character.
84 \L Lowercase till \E.
85 \n (Logical) newline character.
86 \N{} Named (Unicode) character.
87 \p{}, \pP Character with a Unicode property.
88 \P{}, \PP Character without a Unicode property.
89 \Q Quotemeta till \E.
90 \r Return character.
91 \R Generic new line.
92 \s Character class for white space.
93 \S Character class for non white space.
94 \t Tab character.
95 \u Uppercase next character.
96 \U Uppercase till \E.
97 \v Character class for vertical white space.
98 \V Character class for non vertical white space.
99 \w Character class for word characters.
100 \W Character class for non-word characters.
101 \x{}, \x00 Hexadecimal escape sequence.
102 \X Extended Unicode "combining character sequence".
103 \z End of string.
104 \Z End of string.
105
106=head2 Character Escapes
107
108=head3 Fixed characters
109
110A handful of characters have a dedidated I<character escape>. The following
111table shows them, along with their code points (in decimal and hex), their
112ASCII name, the control escape (see below) and a short description.
113
114 Seq. Code Point ASCII Cntr Description.
115 Dec Hex
116 \a 7 07 BEL \cG alarm or bell
117 \b 8 08 BS \cH backspace [1]
118 \e 27 1B ESC \c[ escape character
119 \f 12 0C FF \cL form feed
120 \n 10 0A LF \cJ line feed [2]
121 \r 13 0D CR \cM carriage return
122 \t 9 09 TAB \cI tab
123
124=over 4
125
126=item [1]
127
128C<\b> is only the backspace character inside a character class. Outside a
129character class, C<\b> is a word/non-word boundary.
130
131=item [2]
132
133C<\n> matches a logical newline. Perl will convert between C<\n> and your
134OSses native newline character when reading from or writing to text files.
135
136=back
137
138=head4 Example
139
140 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
141
142=head3 Control characters
143
144C<\c> is used to denote a control character; the character following C<\c>
145is the name of the control character. For instance, C</\cM/> matches the
146character I<control-M> (a carriage return, code point 13). The case of the
147character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same
148character.
149
150Mnemonic: I<c>ontrol character.
151
152=head4 Example
153
154 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
155
156=head3 Named characters
157
158All Unicode characters have a Unicode name, and characters in various scripts
159have names as well. It is even possible to give your own names to characters.
160You can use a character by name by using the C<\N{}> construct; the name of
161the character goes between the curly braces. You do have to C<use charnames>
162to load the names of the characters, otherwise Perl will complain you use
163a name it doesn't know about. For more details, see L<charnames>.
164
165Mnemonic: I<N>amed character.
166
167=head4 Example
168
169 use charnames ':full'; # Loads the Unicode names.
170 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
171
172 use charnames 'Cyrillic'; # Loads Cyrillic names.
173 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
174
175=head3 Octal escapes
176
177Octal escapes consist of a backslash followed by two or three octal digits
178matching the code point of the character you want to use. This allows for
179522 characters (C<\00> up to C<\777>) that can be expressed this way.
180Enough in pre-Unicode days, but most Unicode characters cannot be escaped
181this way.
182
183Note that a character that is expressed as an octal escape is considered
184as a character without special meaning by the regex engine, and will match
185"as is".
186
187=head4 Examples
188
189 $str = "Perl";
190 $str =~ /\120/; # Match, "\120" is "P".
191 $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
192 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
193
194=head4 Caveat
195
196Octal escapes potentially clash with backreferences. They both consist
197of a backslash followed by numbers. So Perl has to use heuristics to
198determine whether it is a backreference or an octal escape. Perl uses
199the following rules:
200
201=over 4
202
203=item 1
204
205If the backslash is followed by a single digit, it's a backrefence.
206
207=item 2
208
209If the first digit following the backslash is a 0, it's an octal escape.
210
211=item 3
212
213If the number following the backslash is N (decimal), and Perl already has
214seen N capture groups, Perl will consider this to be a backreference.
215Otherwise, it will consider it to be an octal escape. Note that if N > 999,
216Perl only takes the first three digits for the octal escape; the rest is
217matched as is.
218
219 my $pat = "(" x 999;
220 $pat .= "a";
221 $pat .= ")" x 999;
222 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
223 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
224 # and \1000 is seen as \100 (a '@') and a '0'.
225
226=back
227
228=head3 Hexadecimal escapes
229
230Hexadecimal escapes start with C<\x> and are then either followed by
231two digit hexadecimal number, or a hexadecimal number of arbitrary length
232surrounded by curly braces. The hexadecimal number is the code point of
233the character you want to express.
234
235Note that a character that is expressed as a hexadecimal escape is considered
236as a character without special meaning by the regex engine, and will match
237"as is".
238
239Mnemonic: heI<x>adecimal.
240
241=head4 Examples
242
243 $str = "Perl";
244 $str =~ /\x50/; # Match, "\x50" is "P".
245 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
246 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
247
248 /\x{2603}\x{2602}/ # Snowman with an umbrella.
249 # The Unicode character 2603 is a snowman,
250 # the Unicode character 2602 is an umbrella.
251 /\x{263B}/ # Black smiling face.
252 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
253
254=head2 Modifiers
255
256A number of backslash sequences have to do with changing the character,
257or characters following them. C<\l> will lowercase the character following
258it, while C<\u> will uppercase the character following it. (They perform
259similar functionality as the functions C<lcfirst> and C<ucfirst>).
260
261To uppercase or lowercase several characters, one might want to use
262C<\L> or C<\U>, which will lowercase/uppercase all characters following
263them, until either the end of the pattern, or the next occurance of
264C<\E>, whatever comes first. They perform similar functionality as the
265functions C<lc> and C<uc> do.
266
267C<\Q> is used to escape all characters following, up to the next C<\E>
268or the end of the pattern. C<\Q> adds a backslash to any character that
269isn't a letter, digit or underscore. This will ensure that any character
270between C<\Q> and C<\E> is matched literally, and will not be interpreted
271by the regexp engine.
272
273Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
274
275=head4 Examples
276
277 $sid = "sid";
278 $greg = "GrEg";
279 $miranda = "(Miranda)";
280 $str =~ /\u$sid/; # Matches 'Sid'
281 $str =~ /\L$greg/; # Matches 'greg'
282 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
283 # had been written as /\(Miranda\)/
284
285=head2 Character classes
286
287Perl regular expressions have a large range of character classes. Some of
288the character classes are written as a backslash sequence. We will briefly
289discuss those here; full details of character classes can be found in
290L<perlrecharclass>.
291
292C<\w> is a character class that matches any I<word> character (letters,
293digits, underscore). C<\d> is a character class that matches any digit,
294while the character class C<\s> matches any white space character.
295New in perl 5.10 are the classes C<\h> and C<\v> which match horizontal
296and vertical white space characters.
297
298The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
299character classes that match any character that isn't a word character,
300digit, white space, horizontal white space or vertical white space.
301
302Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
303
304=head3 Unicode classes
305
306C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
307match a character that matches the given Unicode property; properties
308include things like "letter", or "thai character". Capitalizing the
309sequence to C<\PP> and C<\P{Property}> make the sequence match a character
310that doesn't match the given Unicode property. For more details, see
311L<perlrecharclass/Backslashed sequences> and
312L<perlunicode/Unicode Character Properties>.
313
314Mnemonic: I<p>roperty.
315
316
317=head2 Referencing
318
319If capturing parenthesis are used in a regular expression, we can refer
320to the part of the source string that was matched, and match exactly the
321same thing. (Full details are discussed in L<perlrecapture>). There are
322three ways of refering to such I<backreference>: absolutely, relatively,
323and by name.
324
325=head3 Absolute referencing
326
327A backslash sequence that starts with a backslash and is followed by a
328number is an absolute reference (but be aware of the caveat mentioned above).
329If the number is I<N>, it refers to the Nth set of parenthesis - whatever
330has been matched by that set of parenthesis has to be matched by the C<\N>
331as well.
332
333=head4 Examples
334
335 /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
336 /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
337
338
339=head3 Relative referencing
340
341New in perl 5.10 is different way of refering to capture buffers: C<\g>.
342C<\g> takes a number as argument, with the number in curly braces (the
343braces are optional). If the number (N) does not have a sign, it's a reference
344to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
345C<\g> always refers to a capture group and will never be seen as an octal
346escape). If the number is negative, the reference is relative, refering to
347the Nth group before the C<\g{-N}>.
348
349The big advantage of C<\g{-N}> is that it makes it much easier to write
350patterns with references that can be interpolated in larger patterns,
351even if the larger pattern also contains capture groups.
352
353Mnemonic: I<g>roup.
354
355=head4 Examples
356
357 /(A) # Buffer 1
358 ( # Buffer 2
359 (B) # Buffer 3
360 \g{-1} # Refers to buffer 3 (B)
361 \g{-3} # Refers to buffer 1 (A)
362 )
363 /x; # Matches "ABBA".
364
365 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
366 /$qr$qr/ # Matches 'ababcdcd'.
367
368=head3 Named referencing
369
370Also new in perl 5.10 is the use of named capture buffers, which can be
371referred to by name. This is done with C<\g{name}>, which is a
372backreference to the capture buffer with the name I<name>.
373
374To be compatible with .Net regular expressions, C<\g{name}> may also be
375written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
376
377Note that C<\g{}> has the potential to be ambiguous, as it could be a named
378reference, or an absolute or relative reference (if its argument is numeric).
379However, names are not allowed to start with digits, nor are allowed to
380contain a hyphen, so there is no ambiguity.
381
382=head4 Examples
383
384 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
385 /(?<word>\w+) \k{word}/ # Same.
386 /(?<word>\w+) \k<word>/ # Same.
387 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
388 # Match a four letter palindrome (e.g. "ABBA")
389
390=head2 Assertions
391
392Assertions are conditions that have to be true -- they don't actually
393match parts of the substring. There are six assertions that are written as
394backslash sequences.
395
396=over 4
397
398=item \A
399
400C<\A> only matches at the beginning of the string. If the C</m> modifier
401isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
402modifier is used, then C</^/> matches internal newlines, but the meaning
403of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
404of the string regardless whether the C</m> modifier is used.
405
406=item \z, \Z
407
408C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
409used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
410end of the string, or before the newline at the end of the string. If the
411C</m> modifier is used, then C</$/> matches at internal newlines, but the
412meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
413the end of the string (or just before a trailing newline) regardless whether
414the C</m> modifier is used.
415
416C<\z> is just like C<\Z>, except that it will not match before a trailing
417newline. C<\z> will only match at the end of the string - regardless of the
418modifiers used, and not before a newline.
419
420=item \G
421
422C<\G> is usually only used in combination with the C</g> modifier. If the
423C</g> modifier is used (and the match is done in scalar context), Perl will
424remember where in the source string the last match ended, and the next time,
425it will start the match from where it ended the previous time.
426
427C<\G> matches the point where the previous match ended, or the beginning
428of the string if there was no previous match. See also L<perlremodifiers>.
429
430Mnemonic: I<G>lobal.
431
432=item \b, \B
433
434C<\b> matches at any place between a word and a non-word character; C<\B>
435matches at any place between characters where C<\b> doesn't match. C<\b>
436and C<\B> assume there's a non-word character before the beginning and after
437the end of the source string; so C<\b> will match at the beginning (or end)
438of the source string if the source string begins (or ends) with a word
439character. Otherwise, C<\B> will match.
440
441Mnemonic: I<b>oundary.
442
443=back
444
445=head4 Examples
446
447 "cat" =~ /\Acat/; # Match.
448 "cat" =~ /cat\Z/; # Match.
449 "cat\n" =~ /cat\Z/; # Match.
450 "cat\n" =~ /cat\z/; # No match.
451
452 "cat" =~ /\bcat\b/; # Matches.
453 "cats" =~ /\bcat\b/; # No match.
454 "cat" =~ /\bcat\B/; # No match.
455 "cats" =~ /\bcat\B/; # Match.
456
457 while ("cat dog" =~ /(\w+)/g) {
458 print $1; # Prints 'catdog'
459 }
460 while ("cat dog" =~ /\G(\w+)/g) {
461 print $1; # Prints 'cat'
462 }
463
464=head2 Misc
465
466Here we document the backslash sequences that don't fall in one of the
467categories above. They are:
468
469=over 4
470
471=item \C
472
473C<\C> always matches a single octet, even if the source string is encoded
474in UTF-8 format, and the character to be matched is a multi-octet character.
475C<\C> was introduced in perl 5.6.
476
477Mnemonic: oI<C>tet.
478
479=item \K
480
481This is new in perl 5.10. Anything that is matched left of C<\K> is
482not included in C<$&> - and will not be replaced if the pattern is
483used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
484instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
485
486Mnemonic: I<K>eep.
487
488=item \R
489
490C<\R> matches a I<generic newline>, that is, anything that is considered
491a newline by Unicode. This includes all characters matched by C<\v>
492(vertical white space), and the multi character sequence C<"\x0D\x0A">
493(carriage return followed by a line feed, aka the network newline, or
494the newline used in Windows text files). C<\R> is equivalent with
495C<(?>\x0D\x0A)|\v)>. Since C<\R> can match a more than one character,
496it cannot be put inside a bracketed character class; C</[\R]/> is an error.
497C<\R> is introduced in perl 5.10.
498
499Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>.
500
501=item \X
502
503This matches an extended Unicode I<combining character sequence>, and
504is equivalent to C<< (?>\PM\pM*) >>. C<\PM> matches any character that is
505not considered a Unicode mark character, while C<\pM> matches any character
506that is considered a Unicode mark character; so C<\X> matches any non
507mark character followed by zero or more mark characters. Mark characters
508include (but are not restricted to) I<combining characters> and
509I<vowel signs>.
510
511Mnemonic: eI<X>tended Unicode character.
512
513=back
514
515=head4 Examples
516
517 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
518
519 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
520 $str =~ s/(.)\K\1//g; # Delete duplicated characters.
521
522 "\n" =~ /^\R$/; # Match, \n is a generic newline.
523 "\r" =~ /^\R$/; # Match, \r is a generic newline.
524 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
525
526 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
527
528=cut