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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 | ||
40 | It is however garanteed that backslash or escape sequences never have a | |
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. | |
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 | ||
110 | A handful of characters have a dedidated I<character escape>. The following | |
111 | table shows them, along with their code points (in decimal and hex), their | |
112 | ASCII 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 | ||
128 | C<\b> is only the backspace character inside a character class. Outside a | |
129 | character class, C<\b> is a word/non-word boundary. | |
130 | ||
131 | =item [2] | |
132 | ||
133 | C<\n> matches a logical newline. Perl will convert between C<\n> and your | |
134 | OSses 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 | ||
144 | C<\c> is used to denote a control character; the character following C<\c> | |
145 | is the name of the control character. For instance, C</\cM/> matches the | |
146 | character I<control-M> (a carriage return, code point 13). The case of the | |
147 | character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same | |
148 | character. | |
149 | ||
150 | Mnemonic: 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 | ||
158 | All Unicode characters have a Unicode name, and characters in various scripts | |
159 | have names as well. It is even possible to give your own names to characters. | |
160 | You can use a character by name by using the C<\N{}> construct; the name of | |
161 | the character goes between the curly braces. You do have to C<use charnames> | |
162 | to load the names of the characters, otherwise Perl will complain you use | |
163 | a name it doesn't know about. For more details, see L<charnames>. | |
164 | ||
165 | Mnemonic: 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 | ||
177 | Octal escapes consist of a backslash followed by two or three octal digits | |
178 | matching the code point of the character you want to use. This allows for | |
179 | 522 characters (C<\00> up to C<\777>) that can be expressed this way. | |
180 | Enough in pre-Unicode days, but most Unicode characters cannot be escaped | |
181 | this way. | |
182 | ||
183 | Note that a character that is expressed as an octal escape is considered | |
184 | as 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 | ||
196 | Octal escapes potentially clash with backreferences. They both consist | |
197 | of a backslash followed by numbers. So Perl has to use heuristics to | |
198 | determine whether it is a backreference or an octal escape. Perl uses | |
199 | the following rules: | |
200 | ||
201 | =over 4 | |
202 | ||
203 | =item 1 | |
204 | ||
205 | If the backslash is followed by a single digit, it's a backrefence. | |
206 | ||
207 | =item 2 | |
208 | ||
209 | If the first digit following the backslash is a 0, it's an octal escape. | |
210 | ||
211 | =item 3 | |
212 | ||
213 | If the number following the backslash is N (decimal), and Perl already has | |
214 | seen N capture groups, Perl will consider this to be a backreference. | |
215 | Otherwise, it will consider it to be an octal escape. Note that if N > 999, | |
216 | Perl only takes the first three digits for the octal escape; the rest is | |
217 | matched 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 | ||
230 | Hexadecimal escapes start with C<\x> and are then either followed by | |
231 | two digit hexadecimal number, or a hexadecimal number of arbitrary length | |
232 | surrounded by curly braces. The hexadecimal number is the code point of | |
233 | the character you want to express. | |
234 | ||
235 | Note that a character that is expressed as a hexadecimal escape is considered | |
236 | as a character without special meaning by the regex engine, and will match | |
237 | "as is". | |
238 | ||
239 | Mnemonic: 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 | ||
256 | A number of backslash sequences have to do with changing the character, | |
257 | or characters following them. C<\l> will lowercase the character following | |
258 | it, while C<\u> will uppercase the character following it. (They perform | |
259 | similar functionality as the functions C<lcfirst> and C<ucfirst>). | |
260 | ||
261 | To uppercase or lowercase several characters, one might want to use | |
262 | C<\L> or C<\U>, which will lowercase/uppercase all characters following | |
263 | them, until either the end of the pattern, or the next occurance of | |
264 | C<\E>, whatever comes first. They perform similar functionality as the | |
265 | functions C<lc> and C<uc> do. | |
266 | ||
267 | C<\Q> is used to escape all characters following, up to the next C<\E> | |
268 | or the end of the pattern. C<\Q> adds a backslash to any character that | |
269 | isn't a letter, digit or underscore. This will ensure that any character | |
270 | between C<\Q> and C<\E> is matched literally, and will not be interpreted | |
271 | by the regexp engine. | |
272 | ||
273 | Mnemonic: 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 | ||
287 | Perl regular expressions have a large range of character classes. Some of | |
288 | the character classes are written as a backslash sequence. We will briefly | |
289 | discuss those here; full details of character classes can be found in | |
290 | L<perlrecharclass>. | |
291 | ||
292 | C<\w> is a character class that matches any I<word> character (letters, | |
293 | digits, underscore). C<\d> is a character class that matches any digit, | |
294 | while the character class C<\s> matches any white space character. | |
295 | New in perl 5.10 are the classes C<\h> and C<\v> which match horizontal | |
296 | and vertical white space characters. | |
297 | ||
298 | The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are | |
299 | character classes that match any character that isn't a word character, | |
300 | digit, white space, horizontal white space or vertical white space. | |
301 | ||
302 | Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical. | |
303 | ||
304 | =head3 Unicode classes | |
305 | ||
306 | C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to | |
307 | match a character that matches the given Unicode property; properties | |
308 | include things like "letter", or "thai character". Capitalizing the | |
309 | sequence to C<\PP> and C<\P{Property}> make the sequence match a character | |
310 | that doesn't match the given Unicode property. For more details, see | |
311 | L<perlrecharclass/Backslashed sequences> and | |
312 | L<perlunicode/Unicode Character Properties>. | |
313 | ||
314 | Mnemonic: I<p>roperty. | |
315 | ||
316 | ||
317 | =head2 Referencing | |
318 | ||
319 | If capturing parenthesis are used in a regular expression, we can refer | |
320 | to the part of the source string that was matched, and match exactly the | |
321 | same thing. (Full details are discussed in L<perlrecapture>). There are | |
322 | three ways of refering to such I<backreference>: absolutely, relatively, | |
323 | and by name. | |
324 | ||
325 | =head3 Absolute referencing | |
326 | ||
327 | A backslash sequence that starts with a backslash and is followed by a | |
328 | number is an absolute reference (but be aware of the caveat mentioned above). | |
329 | If the number is I<N>, it refers to the Nth set of parenthesis - whatever | |
330 | has been matched by that set of parenthesis has to be matched by the C<\N> | |
331 | as 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 | ||
341 | New in perl 5.10 is different way of refering to capture buffers: C<\g>. | |
342 | C<\g> takes a number as argument, with the number in curly braces (the | |
343 | braces are optional). If the number (N) does not have a sign, it's a reference | |
344 | to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that | |
345 | C<\g> always refers to a capture group and will never be seen as an octal | |
346 | escape). If the number is negative, the reference is relative, refering to | |
347 | the Nth group before the C<\g{-N}>. | |
348 | ||
349 | The big advantage of C<\g{-N}> is that it makes it much easier to write | |
350 | patterns with references that can be interpolated in larger patterns, | |
351 | even if the larger pattern also contains capture groups. | |
352 | ||
353 | Mnemonic: 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$qr/ # Matches 'ababcdcd'. | |
367 | ||
368 | =head3 Named referencing | |
369 | ||
370 | Also new in perl 5.10 is the use of named capture buffers, which can be | |
371 | referred to by name. This is done with C<\g{name}>, which is a | |
372 | backreference to the capture buffer with the name I<name>. | |
373 | ||
374 | To be compatible with .Net regular expressions, C<\g{name}> may also be | |
375 | written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. | |
376 | ||
377 | Note that C<\g{}> has the potential to be ambiguous, as it could be a named | |
378 | reference, or an absolute or relative reference (if its argument is numeric). | |
379 | However, names are not allowed to start with digits, nor are allowed to | |
380 | contain a hyphen, so there is no ambiguity. | |
381 | ||
382 | =head4 Examples | |
383 | ||
384 | /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat") | |
385 | /(?<word>\w+) \k{word}/ # Same. | |
386 | /(?<word>\w+) \k<word>/ # Same. | |
387 | /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/ | |
388 | # Match a four letter palindrome (e.g. "ABBA") | |
389 | ||
390 | =head2 Assertions | |
391 | ||
392 | Assertions are conditions that have to be true -- they don't actually | |
393 | match parts of the substring. There are six assertions that are written as | |
394 | backslash sequences. | |
395 | ||
396 | =over 4 | |
397 | ||
398 | =item \A | |
399 | ||
400 | C<\A> only matches at the beginning of the string. If the C</m> modifier | |
401 | isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m> | |
402 | modifier is used, then C</^/> matches internal newlines, but the meaning | |
403 | of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning | |
404 | of the string regardless whether the C</m> modifier is used. | |
405 | ||
406 | =item \z, \Z | |
407 | ||
408 | C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't | |
409 | used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the | |
410 | end of the string, or before the newline at the end of the string. If the | |
411 | C</m> modifier is used, then C</$/> matches at internal newlines, but the | |
412 | meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at | |
413 | the end of the string (or just before a trailing newline) regardless whether | |
414 | the C</m> modifier is used. | |
415 | ||
416 | C<\z> is just like C<\Z>, except that it will not match before a trailing | |
417 | newline. C<\z> will only match at the end of the string - regardless of the | |
418 | modifiers used, and not before a newline. | |
419 | ||
420 | =item \G | |
421 | ||
422 | C<\G> is usually only used in combination with the C</g> modifier. If the | |
423 | C</g> modifier is used (and the match is done in scalar context), Perl will | |
424 | remember where in the source string the last match ended, and the next time, | |
425 | it will start the match from where it ended the previous time. | |
426 | ||
427 | C<\G> matches the point where the previous match ended, or the beginning | |
428 | of the string if there was no previous match. See also L<perlremodifiers>. | |
429 | ||
430 | Mnemonic: I<G>lobal. | |
431 | ||
432 | =item \b, \B | |
433 | ||
434 | C<\b> matches at any place between a word and a non-word character; C<\B> | |
435 | matches at any place between characters where C<\b> doesn't match. C<\b> | |
436 | and C<\B> assume there's a non-word character before the beginning and after | |
437 | the end of the source string; so C<\b> will match at the beginning (or end) | |
438 | of the source string if the source string begins (or ends) with a word | |
439 | character. Otherwise, C<\B> will match. | |
440 | ||
441 | Mnemonic: 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 $1; # Prints 'cat' | |
462 | } | |
463 | ||
464 | =head2 Misc | |
465 | ||
466 | Here we document the backslash sequences that don't fall in one of the | |
467 | categories above. They are: | |
468 | ||
469 | =over 4 | |
470 | ||
471 | =item \C | |
472 | ||
473 | C<\C> always matches a single octet, even if the source string is encoded | |
474 | in UTF-8 format, and the character to be matched is a multi-octet character. | |
475 | C<\C> was introduced in perl 5.6. | |
476 | ||
477 | Mnemonic: oI<C>tet. | |
478 | ||
479 | =item \K | |
480 | ||
481 | This is new in perl 5.10. Anything that is matched left of C<\K> is | |
482 | not included in C<$&> - and will not be replaced if the pattern is | |
483 | used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x> | |
484 | instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>. | |
485 | ||
486 | Mnemonic: I<K>eep. | |
487 | ||
488 | =item \R | |
489 | ||
490 | C<\R> matches a I<generic newline>, that is, anything that is considered | |
491 | a newline by Unicode. This includes all characters matched by C<\v> | |
492 | (vertical white space), and the multi character sequence C<"\x0D\x0A"> | |
493 | (carriage return followed by a line feed, aka the network newline, or | |
494 | the newline used in Windows text files). C<\R> is equivalent with | |
495 | C<(?>\x0D\x0A)|\v)>. Since C<\R> can match a more than one character, | |
496 | it cannot be put inside a bracketed character class; C</[\R]/> is an error. | |
497 | C<\R> is introduced in perl 5.10. | |
498 | ||
499 | Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>. | |
500 | ||
501 | =item \X | |
502 | ||
503 | This matches an extended Unicode I<combining character sequence>, and | |
504 | is equivalent to C<< (?>\PM\pM*) >>. C<\PM> matches any character that is | |
505 | not considered a Unicode mark character, while C<\pM> matches any character | |
506 | that is considered a Unicode mark character; so C<\X> matches any non | |
507 | mark character followed by zero or more mark characters. Mark characters | |
508 | include (but are not restricted to) I<combining characters> and | |
509 | I<vowel signs>. | |
510 | ||
511 | Mnemonic: eI<X>tended Unicode character. | |
512 | ||
513 | =back | |
514 | ||
515 | =head4 Examples | |
516 | ||
517 | "\x{256}" =~ /^\C\C$/; # 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 |