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1 | =head1 NAME |
2 | |
3 | perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | The top level documentation about Perl regular expressions |
8 | is found in L<perlre>. |
9 | |
10 | This document describes all backslash and escape sequences. After |
11 | explaining the role of the backslash, it lists all the sequences that have |
12 | a special meaning in Perl regular expressions (in alphabetical order), |
13 | then describes each of them. |
14 | |
15 | Most sequences are described in detail in different documents; the primary |
16 | purpose of this document is to have a quick reference guide describing all |
17 | backslash and escape sequences. |
18 | |
19 | |
20 | =head2 The backslash |
21 | |
22 | In a regular expression, the backslash can perform one of two tasks: |
23 | it either takes away the special meaning of the character following it |
24 | (for instance, C<\|> matches a vertical bar, it's not an alternation), |
25 | or it is the start of a backslash or escape sequence. |
26 | |
27 | The rules determining what it is are quite simple: if the character |
28 | following the backslash is a punctuation (non-word) character (that is, |
29 | anything that is not a letter, digit or underscore), then the backslash |
30 | just takes away the special meaning (if any) of the character following |
31 | it. |
32 | |
33 | If the character following the backslash is a letter or a digit, then the |
34 | sequence may be special; if so, it's listed below. A few letters have not |
35 | been used yet, and escaping them with a backslash is safe for now, but a |
36 | future version of Perl may assign a special meaning to it. However, if you |
37 | have warnings turned on, Perl will issue a warning if you use such a sequence. |
38 | [1]. |
39 | |
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40 | It is however guaranteed that backslash or escape sequences never have a |
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41 | punctuation character following the backslash, not now, and not in a future |
42 | version of Perl 5. So it is safe to put a backslash in front of a non-word |
43 | character. |
44 | |
45 | Note that the backslash itself is special; if you want to match a backslash, |
46 | you have to escape the backslash with a backslash: C</\\/> matches a single |
47 | backslash. |
48 | |
49 | =over 4 |
50 | |
51 | =item [1] |
52 | |
53 | There is one exception. If you use an alphanumerical character as the |
54 | delimiter of your pattern (which you probably shouldn't do for readability |
55 | reasons), you will have to escape the delimiter if you want to match |
56 | it. Perl won't warn then. See also L<perlop/Gory details of parsing |
57 | quoted 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. |
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86 | \N Any character but newline. |
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87 | \N{} Named (Unicode) character. |
88 | \p{}, \pP Character with a Unicode property. |
89 | \P{}, \PP Character without a Unicode property. |
90 | \Q Quotemeta till \E. |
91 | \r Return character. |
92 | \R Generic new line. |
93 | \s Character class for white space. |
94 | \S Character class for non white space. |
95 | \t Tab character. |
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96 | \u Titlecase next character. |
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97 | \U Uppercase till \E. |
98 | \v Character class for vertical white space. |
99 | \V Character class for non vertical white space. |
100 | \w Character class for word characters. |
101 | \W Character class for non-word characters. |
102 | \x{}, \x00 Hexadecimal escape sequence. |
103 | \X Extended Unicode "combining character sequence". |
104 | \z End of string. |
105 | \Z End of string. |
106 | |
107 | =head2 Character Escapes |
108 | |
109 | =head3 Fixed characters |
110 | |
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111 | A handful of characters have a dedicated I<character escape>. The following |
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112 | table shows them, along with their code points (in decimal and hex), their |
113 | ASCII name, the control escape (see below) and a short description. |
114 | |
115 | Seq. Code Point ASCII Cntr Description. |
116 | Dec Hex |
117 | \a 7 07 BEL \cG alarm or bell |
118 | \b 8 08 BS \cH backspace [1] |
119 | \e 27 1B ESC \c[ escape character |
120 | \f 12 0C FF \cL form feed |
121 | \n 10 0A LF \cJ line feed [2] |
122 | \r 13 0D CR \cM carriage return |
123 | \t 9 09 TAB \cI tab |
124 | |
125 | =over 4 |
126 | |
127 | =item [1] |
128 | |
129 | C<\b> is only the backspace character inside a character class. Outside a |
130 | character class, C<\b> is a word/non-word boundary. |
131 | |
132 | =item [2] |
133 | |
134 | C<\n> matches a logical newline. Perl will convert between C<\n> and your |
135 | OSses native newline character when reading from or writing to text files. |
136 | |
137 | =back |
138 | |
139 | =head4 Example |
140 | |
141 | $str =~ /\t/; # Matches if $str contains a (horizontal) tab. |
142 | |
143 | =head3 Control characters |
144 | |
145 | C<\c> is used to denote a control character; the character following C<\c> |
146 | is the name of the control character. For instance, C</\cM/> matches the |
147 | character I<control-M> (a carriage return, code point 13). The case of the |
148 | character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same |
149 | character. |
150 | |
151 | Mnemonic: I<c>ontrol character. |
152 | |
153 | =head4 Example |
154 | |
155 | $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K). |
156 | |
157 | =head3 Named characters |
158 | |
159 | All Unicode characters have a Unicode name, and characters in various scripts |
160 | have names as well. It is even possible to give your own names to characters. |
161 | You can use a character by name by using the C<\N{}> construct; the name of |
162 | the character goes between the curly braces. You do have to C<use charnames> |
163 | to load the names of the characters, otherwise Perl will complain you use |
164 | a name it doesn't know about. For more details, see L<charnames>. |
165 | |
166 | Mnemonic: I<N>amed character. |
167 | |
168 | =head4 Example |
169 | |
170 | use charnames ':full'; # Loads the Unicode names. |
171 | $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character |
172 | |
173 | use charnames 'Cyrillic'; # Loads Cyrillic names. |
174 | $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA". |
175 | |
176 | =head3 Octal escapes |
177 | |
178 | Octal escapes consist of a backslash followed by two or three octal digits |
179 | matching the code point of the character you want to use. This allows for |
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180 | 512 characters (C<\00> up to C<\777>) that can be expressed this way. |
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181 | Enough in pre-Unicode days, but most Unicode characters cannot be escaped |
182 | this way. |
183 | |
184 | Note that a character that is expressed as an octal escape is considered |
185 | as a character without special meaning by the regex engine, and will match |
186 | "as is". |
187 | |
188 | =head4 Examples |
189 | |
190 | $str = "Perl"; |
191 | $str =~ /\120/; # Match, "\120" is "P". |
192 | $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once. |
193 | $str =~ /P\053/; # No match, "\053" is "+" and taken literally. |
194 | |
195 | =head4 Caveat |
196 | |
197 | Octal escapes potentially clash with backreferences. They both consist |
198 | of a backslash followed by numbers. So Perl has to use heuristics to |
199 | determine whether it is a backreference or an octal escape. Perl uses |
200 | the following rules: |
201 | |
202 | =over 4 |
203 | |
204 | =item 1 |
205 | |
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206 | If the backslash is followed by a single digit, it's a backreference. |
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207 | |
208 | =item 2 |
209 | |
210 | If the first digit following the backslash is a 0, it's an octal escape. |
211 | |
212 | =item 3 |
213 | |
214 | If the number following the backslash is N (decimal), and Perl already has |
215 | seen N capture groups, Perl will consider this to be a backreference. |
216 | Otherwise, it will consider it to be an octal escape. Note that if N > 999, |
217 | Perl only takes the first three digits for the octal escape; the rest is |
218 | matched as is. |
219 | |
220 | my $pat = "(" x 999; |
221 | $pat .= "a"; |
222 | $pat .= ")" x 999; |
223 | /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups. |
224 | /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups |
225 | # and \1000 is seen as \100 (a '@') and a '0'. |
226 | |
227 | =back |
228 | |
229 | =head3 Hexadecimal escapes |
230 | |
231 | Hexadecimal escapes start with C<\x> and are then either followed by |
232 | two digit hexadecimal number, or a hexadecimal number of arbitrary length |
233 | surrounded by curly braces. The hexadecimal number is the code point of |
234 | the character you want to express. |
235 | |
236 | Note that a character that is expressed as a hexadecimal escape is considered |
237 | as a character without special meaning by the regex engine, and will match |
238 | "as is". |
239 | |
240 | Mnemonic: heI<x>adecimal. |
241 | |
242 | =head4 Examples |
243 | |
244 | $str = "Perl"; |
245 | $str =~ /\x50/; # Match, "\x50" is "P". |
246 | $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once. |
247 | $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally. |
248 | |
249 | /\x{2603}\x{2602}/ # Snowman with an umbrella. |
250 | # The Unicode character 2603 is a snowman, |
251 | # the Unicode character 2602 is an umbrella. |
252 | /\x{263B}/ # Black smiling face. |
253 | /\x{263b}/ # Same, the hex digits A - F are case insensitive. |
254 | |
255 | =head2 Modifiers |
256 | |
257 | A number of backslash sequences have to do with changing the character, |
258 | or characters following them. C<\l> will lowercase the character following |
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259 | it, while C<\u> will uppercase (or, more accurately, titlecase) the |
260 | character following it. (They perform similar functionality as the |
261 | functions C<lcfirst> and C<ucfirst>). |
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262 | |
263 | To uppercase or lowercase several characters, one might want to use |
264 | C<\L> or C<\U>, which will lowercase/uppercase all characters following |
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265 | them, until either the end of the pattern, or the next occurrence of |
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266 | C<\E>, whatever comes first. They perform similar functionality as the |
267 | functions C<lc> and C<uc> do. |
268 | |
269 | C<\Q> is used to escape all characters following, up to the next C<\E> |
270 | or the end of the pattern. C<\Q> adds a backslash to any character that |
271 | isn't a letter, digit or underscore. This will ensure that any character |
272 | between C<\Q> and C<\E> is matched literally, and will not be interpreted |
273 | by the regexp engine. |
274 | |
275 | Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd. |
276 | |
277 | =head4 Examples |
278 | |
279 | $sid = "sid"; |
280 | $greg = "GrEg"; |
281 | $miranda = "(Miranda)"; |
282 | $str =~ /\u$sid/; # Matches 'Sid' |
283 | $str =~ /\L$greg/; # Matches 'greg' |
284 | $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern |
285 | # had been written as /\(Miranda\)/ |
286 | |
287 | =head2 Character classes |
288 | |
289 | Perl regular expressions have a large range of character classes. Some of |
290 | the character classes are written as a backslash sequence. We will briefly |
291 | discuss those here; full details of character classes can be found in |
292 | L<perlrecharclass>. |
293 | |
294 | C<\w> is a character class that matches any I<word> character (letters, |
295 | digits, underscore). C<\d> is a character class that matches any digit, |
296 | while the character class C<\s> matches any white space character. |
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297 | New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal |
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298 | and vertical white space characters. |
299 | |
300 | The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are |
301 | character classes that match any character that isn't a word character, |
302 | digit, white space, horizontal white space or vertical white space. |
303 | |
304 | Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical. |
305 | |
306 | =head3 Unicode classes |
307 | |
308 | C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to |
309 | match a character that matches the given Unicode property; properties |
310 | include things like "letter", or "thai character". Capitalizing the |
311 | sequence to C<\PP> and C<\P{Property}> make the sequence match a character |
312 | that doesn't match the given Unicode property. For more details, see |
313 | L<perlrecharclass/Backslashed sequences> and |
314 | L<perlunicode/Unicode Character Properties>. |
315 | |
316 | Mnemonic: I<p>roperty. |
317 | |
318 | |
319 | =head2 Referencing |
320 | |
321 | If capturing parenthesis are used in a regular expression, we can refer |
322 | to the part of the source string that was matched, and match exactly the |
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323 | same thing. There are three ways of referring to such I<backreference>: |
324 | absolutely, relatively, and by name. |
325 | |
326 | =for later add link to perlrecapture |
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327 | |
328 | =head3 Absolute referencing |
329 | |
330 | A backslash sequence that starts with a backslash and is followed by a |
331 | number is an absolute reference (but be aware of the caveat mentioned above). |
332 | If the number is I<N>, it refers to the Nth set of parenthesis - whatever |
333 | has been matched by that set of parenthesis has to be matched by the C<\N> |
334 | as well. |
335 | |
336 | =head4 Examples |
337 | |
338 | /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat"). |
339 | /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA"). |
340 | |
341 | |
342 | =head3 Relative referencing |
343 | |
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344 | New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>. |
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345 | C<\g> takes a number as argument, with the number in curly braces (the |
346 | braces are optional). If the number (N) does not have a sign, it's a reference |
347 | to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that |
348 | C<\g> always refers to a capture group and will never be seen as an octal |
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349 | escape). If the number is negative, the reference is relative, referring to |
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350 | the Nth group before the C<\g{-N}>. |
351 | |
352 | The big advantage of C<\g{-N}> is that it makes it much easier to write |
353 | patterns with references that can be interpolated in larger patterns, |
354 | even if the larger pattern also contains capture groups. |
355 | |
356 | Mnemonic: I<g>roup. |
357 | |
358 | =head4 Examples |
359 | |
360 | /(A) # Buffer 1 |
361 | ( # Buffer 2 |
362 | (B) # Buffer 3 |
363 | \g{-1} # Refers to buffer 3 (B) |
364 | \g{-3} # Refers to buffer 1 (A) |
365 | ) |
366 | /x; # Matches "ABBA". |
367 | |
368 | my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc. |
369 | /$qr$qr/ # Matches 'ababcdcd'. |
370 | |
371 | =head3 Named referencing |
372 | |
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373 | Also new in perl 5.10.0 is the use of named capture buffers, which can be |
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374 | referred to by name. This is done with C<\g{name}>, which is a |
375 | backreference to the capture buffer with the name I<name>. |
376 | |
377 | To be compatible with .Net regular expressions, C<\g{name}> may also be |
378 | written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. |
379 | |
380 | Note that C<\g{}> has the potential to be ambiguous, as it could be a named |
381 | reference, or an absolute or relative reference (if its argument is numeric). |
382 | However, names are not allowed to start with digits, nor are allowed to |
383 | contain a hyphen, so there is no ambiguity. |
384 | |
385 | =head4 Examples |
386 | |
387 | /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat") |
388 | /(?<word>\w+) \k{word}/ # Same. |
389 | /(?<word>\w+) \k<word>/ # Same. |
390 | /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/ |
391 | # Match a four letter palindrome (e.g. "ABBA") |
392 | |
393 | =head2 Assertions |
394 | |
395 | Assertions are conditions that have to be true -- they don't actually |
396 | match parts of the substring. There are six assertions that are written as |
397 | backslash sequences. |
398 | |
399 | =over 4 |
400 | |
401 | =item \A |
402 | |
403 | C<\A> only matches at the beginning of the string. If the C</m> modifier |
404 | isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m> |
405 | modifier is used, then C</^/> matches internal newlines, but the meaning |
406 | of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning |
407 | of the string regardless whether the C</m> modifier is used. |
408 | |
409 | =item \z, \Z |
410 | |
411 | C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't |
412 | used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the |
413 | end of the string, or before the newline at the end of the string. If the |
414 | C</m> modifier is used, then C</$/> matches at internal newlines, but the |
415 | meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at |
416 | the end of the string (or just before a trailing newline) regardless whether |
417 | the C</m> modifier is used. |
418 | |
419 | C<\z> is just like C<\Z>, except that it will not match before a trailing |
420 | newline. C<\z> will only match at the end of the string - regardless of the |
421 | modifiers used, and not before a newline. |
422 | |
423 | =item \G |
424 | |
425 | C<\G> is usually only used in combination with the C</g> modifier. If the |
426 | C</g> modifier is used (and the match is done in scalar context), Perl will |
427 | remember where in the source string the last match ended, and the next time, |
428 | it will start the match from where it ended the previous time. |
429 | |
430 | C<\G> matches the point where the previous match ended, or the beginning |
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431 | of the string if there was no previous match. |
432 | |
433 | =for later add link to perlremodifiers |
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434 | |
435 | Mnemonic: I<G>lobal. |
436 | |
437 | =item \b, \B |
438 | |
439 | C<\b> matches at any place between a word and a non-word character; C<\B> |
440 | matches at any place between characters where C<\b> doesn't match. C<\b> |
441 | and C<\B> assume there's a non-word character before the beginning and after |
442 | the end of the source string; so C<\b> will match at the beginning (or end) |
443 | of the source string if the source string begins (or ends) with a word |
444 | character. Otherwise, C<\B> will match. |
445 | |
446 | Mnemonic: I<b>oundary. |
447 | |
448 | =back |
449 | |
450 | =head4 Examples |
451 | |
452 | "cat" =~ /\Acat/; # Match. |
453 | "cat" =~ /cat\Z/; # Match. |
454 | "cat\n" =~ /cat\Z/; # Match. |
455 | "cat\n" =~ /cat\z/; # No match. |
456 | |
457 | "cat" =~ /\bcat\b/; # Matches. |
458 | "cats" =~ /\bcat\b/; # No match. |
459 | "cat" =~ /\bcat\B/; # No match. |
460 | "cats" =~ /\bcat\B/; # Match. |
461 | |
462 | while ("cat dog" =~ /(\w+)/g) { |
463 | print $1; # Prints 'catdog' |
464 | } |
465 | while ("cat dog" =~ /\G(\w+)/g) { |
466 | print $1; # Prints 'cat' |
467 | } |
468 | |
469 | =head2 Misc |
470 | |
471 | Here we document the backslash sequences that don't fall in one of the |
472 | categories above. They are: |
473 | |
474 | =over 4 |
475 | |
476 | =item \C |
477 | |
478 | C<\C> always matches a single octet, even if the source string is encoded |
479 | in UTF-8 format, and the character to be matched is a multi-octet character. |
480 | C<\C> was introduced in perl 5.6. |
481 | |
482 | Mnemonic: oI<C>tet. |
483 | |
484 | =item \K |
485 | |
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486 | This is new in perl 5.10.0. Anything that is matched left of C<\K> is |
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487 | not included in C<$&> - and will not be replaced if the pattern is |
488 | used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x> |
489 | instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>. |
490 | |
491 | Mnemonic: I<K>eep. |
492 | |
493 | =item \R |
494 | |
495 | C<\R> matches a I<generic newline>, that is, anything that is considered |
496 | a newline by Unicode. This includes all characters matched by C<\v> |
497 | (vertical white space), and the multi character sequence C<"\x0D\x0A"> |
498 | (carriage return followed by a line feed, aka the network newline, or |
499 | the newline used in Windows text files). C<\R> is equivalent with |
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500 | C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a more than one character, |
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501 | it cannot be put inside a bracketed character class; C</[\R]/> is an error. |
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502 | C<\R> was introduced in perl 5.10.0. |
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503 | |
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504 | Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>, |
505 | and more importantly because Unicode recommends such a regular expression |
506 | metacharacter, and suggests C<\R> as the notation. |
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507 | |
508 | =item \X |
509 | |
510 | This matches an extended Unicode I<combining character sequence>, and |
511 | is equivalent to C<< (?>\PM\pM*) >>. C<\PM> matches any character that is |
512 | not considered a Unicode mark character, while C<\pM> matches any character |
513 | that is considered a Unicode mark character; so C<\X> matches any non |
514 | mark character followed by zero or more mark characters. Mark characters |
515 | include (but are not restricted to) I<combining characters> and |
516 | I<vowel signs>. |
517 | |
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518 | C<\X> matches quite well what normal (non-Unicode-programmer) usage |
519 | would consider a single character: for example a base character |
520 | (the C<\PM> above), for example a letter, followed by zero or more |
521 | diacritics, which are I<combining characters> (the C<\pM*> above). |
522 | |
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523 | Mnemonic: eI<X>tended Unicode character. |
524 | |
525 | =back |
526 | |
527 | =head4 Examples |
528 | |
529 | "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8. |
530 | |
531 | $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'. |
532 | $str =~ s/(.)\K\1//g; # Delete duplicated characters. |
533 | |
534 | "\n" =~ /^\R$/; # Match, \n is a generic newline. |
535 | "\r" =~ /^\R$/; # Match, \r is a generic newline. |
536 | "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline. |
537 | |
538 | "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above. |
539 | |
540 | =cut |