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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 | ||
8a118206 RGS |
19 | =head2 The backslash |
20 | ||
21 | In a regular expression, the backslash can perform one of two tasks: | |
22 | it either takes away the special meaning of the character following it | |
23 | (for instance, C<\|> matches a vertical bar, it's not an alternation), | |
24 | or it is the start of a backslash or escape sequence. | |
25 | ||
26 | The rules determining what it is are quite simple: if the character | |
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27 | following the backslash is an ASCII punctuation (non-word) character (that is, |
28 | anything that is not a letter, digit or underscore), then the backslash just | |
29 | takes away the special meaning (if any) of the character following it. | |
30 | ||
31 | If the character following the backslash is an ASCII letter or an ASCII digit, | |
32 | then the sequence may be special; if so, it's listed below. A few letters have | |
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33 | not been used yet, so escaping them with a backslash doesn't change them to be |
34 | special. A future version of Perl may assign a special meaning to them, so if | |
35 | you have warnings turned on, Perl will issue a warning if you use such a | |
36 | sequence. [1]. | |
8a118206 | 37 | |
e2cb52ee | 38 | It is however guaranteed that backslash or escape sequences never have a |
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39 | punctuation character following the backslash, not now, and not in a future |
40 | version of Perl 5. So it is safe to put a backslash in front of a non-word | |
41 | character. | |
42 | ||
43 | Note that the backslash itself is special; if you want to match a backslash, | |
44 | you have to escape the backslash with a backslash: C</\\/> matches a single | |
45 | backslash. | |
46 | ||
47 | =over 4 | |
48 | ||
49 | =item [1] | |
50 | ||
51 | There is one exception. If you use an alphanumerical character as the | |
52 | delimiter of your pattern (which you probably shouldn't do for readability | |
53 | reasons), you will have to escape the delimiter if you want to match | |
54 | it. Perl won't warn then. See also L<perlop/Gory details of parsing | |
55 | quoted constructs>. | |
56 | ||
57 | =back | |
58 | ||
59 | ||
60 | =head2 All the sequences and escapes | |
61 | ||
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62 | Those not usable within a bracketed character class (like C<[\da-z]>) are marked |
63 | as C<Not in [].> | |
64 | ||
8a118206 | 65 | \000 Octal escape sequence. |
df225385 | 66 | \1 Absolute backreference. Not in []. |
8a118206 | 67 | \a Alarm or bell. |
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68 | \A Beginning of string. Not in []. |
69 | \b Word/non-word boundary. (Backspace in []). | |
70 | \B Not a word/non-word boundary. Not in []. | |
4948b50f | 71 | \cX Control-X |
df225385 | 72 | \C Single octet, even under UTF-8. Not in []. |
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73 | \d Character class for digits. |
74 | \D Character class for non-digits. | |
75 | \e Escape character. | |
df225385 | 76 | \E Turn off \Q, \L and \U processing. Not in []. |
8a118206 | 77 | \f Form feed. |
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78 | \g{}, \g1 Named, absolute or relative backreference. Not in []. |
79 | \G Pos assertion. Not in []. | |
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80 | \h Character class for horizontal whitespace. |
81 | \H Character class for non horizontal whitespace. | |
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82 | \k{}, \k<>, \k'' Named backreference. Not in []. |
83 | \K Keep the stuff left of \K. Not in []. | |
84 | \l Lowercase next character. Not in []. | |
85 | \L Lowercase till \E. Not in []. | |
8a118206 | 86 | \n (Logical) newline character. |
b3b85878 | 87 | \N Any character but newline. Experimental. Not in []. |
e526e8bb | 88 | \N{} Named or numbered (Unicode) character. |
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89 | \p{}, \pP Character with the given Unicode property. |
90 | \P{}, \PP Character without the given Unicode property. | |
df225385 | 91 | \Q Quotemeta till \E. Not in []. |
8a118206 | 92 | \r Return character. |
df225385 | 93 | \R Generic new line. Not in []. |
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94 | \s Character class for whitespace. |
95 | \S Character class for non whitespace. | |
8a118206 | 96 | \t Tab character. |
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97 | \u Titlecase next character. Not in []. |
98 | \U Uppercase till \E. Not in []. | |
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99 | \v Character class for vertical whitespace. |
100 | \V Character class for non vertical whitespace. | |
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101 | \w Character class for word characters. |
102 | \W Character class for non-word characters. | |
103 | \x{}, \x00 Hexadecimal escape sequence. | |
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104 | \X Unicode "extended grapheme cluster". Not in []. |
105 | \z End of string. Not in []. | |
106 | \Z End of string. Not in []. | |
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107 | |
108 | =head2 Character Escapes | |
109 | ||
110 | =head3 Fixed characters | |
111 | ||
e2cb52ee | 112 | A handful of characters have a dedicated I<character escape>. The following |
58151fe4 | 113 | table shows them, along with their ASCII code points (in decimal and hex), |
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114 | their ASCII name, the control escape on ASCII platforms and a short |
115 | description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.) | |
8a118206 | 116 | |
4948b50f | 117 | Seq. Code Point ASCII Cntrl Description. |
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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 | ||
131 | C<\b> is only the backspace character inside a character class. Outside a | |
132 | character class, C<\b> is a word/non-word boundary. | |
133 | ||
134 | =item [2] | |
135 | ||
136 | C<\n> matches a logical newline. Perl will convert between C<\n> and your | |
137 | OSses 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 | ||
147 | C<\c> is used to denote a control character; the character following C<\c> | |
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148 | determines the value of the construct. For example the value of C<\cA> is |
149 | C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc. | |
150 | The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete | |
151 | list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in | |
152 | L<perlebcdic/OPERATOR DIFFERENCES>. | |
153 | ||
154 | Note that C<\c\> alone at the end of a regular expression (or doubled-quoted | |
155 | string) is not valid. The backslash must be followed by another character. | |
156 | That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>. | |
157 | ||
158 | To write platform-independent code, you must use C<\N{I<NAME>}> instead, like | |
159 | C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>. | |
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160 | |
161 | Mnemonic: I<c>ontrol character. | |
162 | ||
163 | =head4 Example | |
164 | ||
165 | $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K). | |
166 | ||
e526e8bb | 167 | =head3 Named or numbered characters |
8a118206 | 168 | |
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169 | All Unicode characters have a Unicode name and numeric ordinal value. Use the |
170 | C<\N{}> construct to specify a character by either of these values. | |
171 | ||
172 | To specify by name, the name of the character goes between the curly braces. | |
173 | In this case, you have to C<use charnames> to load the Unicode names of the | |
174 | characters, otherwise Perl will complain. | |
175 | ||
176 | To specify by Unicode ordinal number, use the form | |
177 | C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in | |
178 | hexadecimal that gives the ordinal number that Unicode has assigned to the | |
179 | desired character. It is customary (but not required) to use leading zeros to | |
180 | pad the number to 4 digits. Thus C<\N{U+0041}> means | |
181 | C<Latin Capital Letter A>, and you will rarely see it written without the two | |
182 | leading zeros. C<\N{U+0041}> means C<A> even on EBCDIC machines (where the | |
183 | ordinal value of C<A> is not 0x41). | |
184 | ||
185 | It is even possible to give your own names to characters, and even to short | |
186 | sequences of characters. For details, see L<charnames>. | |
8a118206 | 187 | |
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188 | (There is an expanded internal form that you may see in debug output: |
189 | C<\N{U+I<wide hex character>.I<wide hex character>...}>. | |
190 | The C<...> means any number of these I<wide hex character>s separated by dots. | |
191 | This represents the sequence formed by the characters. This is an internal | |
192 | form only, subject to change, and you should not try to use it yourself.) | |
193 | ||
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194 | Mnemonic: I<N>amed character. |
195 | ||
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196 | Note that a character that is expressed as a named or numbered character is |
197 | considered as a character without special meaning by the regex engine, and will | |
198 | match "as is". | |
df225385 | 199 | |
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200 | =head4 Example |
201 | ||
202 | use charnames ':full'; # Loads the Unicode names. | |
203 | $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character | |
204 | ||
205 | use charnames 'Cyrillic'; # Loads Cyrillic names. | |
206 | $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA". | |
207 | ||
208 | =head3 Octal escapes | |
209 | ||
210 | Octal escapes consist of a backslash followed by two or three octal digits | |
211 | matching the code point of the character you want to use. This allows for | |
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212 | 512 characters (C<\00> up to C<\777>) that can be expressed this way (but |
213 | anything above C<\377> is deprecated). | |
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214 | Enough in pre-Unicode days, but most Unicode characters cannot be escaped |
215 | this way. | |
216 | ||
217 | Note that a character that is expressed as an octal escape is considered | |
218 | as a character without special meaning by the regex engine, and will match | |
219 | "as is". | |
220 | ||
58151fe4 | 221 | =head4 Examples (assuming an ASCII platform) |
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222 | |
223 | $str = "Perl"; | |
224 | $str =~ /\120/; # Match, "\120" is "P". | |
225 | $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once. | |
226 | $str =~ /P\053/; # No match, "\053" is "+" and taken literally. | |
227 | ||
228 | =head4 Caveat | |
229 | ||
230 | Octal escapes potentially clash with backreferences. They both consist | |
231 | of a backslash followed by numbers. So Perl has to use heuristics to | |
232 | determine whether it is a backreference or an octal escape. Perl uses | |
233 | the following rules: | |
234 | ||
235 | =over 4 | |
236 | ||
237 | =item 1 | |
238 | ||
353c6505 | 239 | If the backslash is followed by a single digit, it's a backreference. |
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240 | |
241 | =item 2 | |
242 | ||
243 | If the first digit following the backslash is a 0, it's an octal escape. | |
244 | ||
245 | =item 3 | |
246 | ||
247 | If the number following the backslash is N (decimal), and Perl already has | |
248 | seen N capture groups, Perl will consider this to be a backreference. | |
249 | Otherwise, it will consider it to be an octal escape. Note that if N > 999, | |
250 | Perl only takes the first three digits for the octal escape; the rest is | |
251 | matched as is. | |
252 | ||
253 | my $pat = "(" x 999; | |
254 | $pat .= "a"; | |
255 | $pat .= ")" x 999; | |
256 | /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups. | |
257 | /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups | |
258 | # and \1000 is seen as \100 (a '@') and a '0'. | |
259 | ||
260 | =back | |
261 | ||
262 | =head3 Hexadecimal escapes | |
263 | ||
58151fe4 | 264 | Hexadecimal escapes start with C<\x> and are then either followed by a |
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265 | two digit hexadecimal number, or a hexadecimal number of arbitrary length |
266 | surrounded by curly braces. The hexadecimal number is the code point of | |
267 | the character you want to express. | |
268 | ||
269 | Note that a character that is expressed as a hexadecimal escape is considered | |
270 | as a character without special meaning by the regex engine, and will match | |
271 | "as is". | |
272 | ||
273 | Mnemonic: heI<x>adecimal. | |
274 | ||
9f5650a8 | 275 | =head4 Examples (assuming an ASCII platform) |
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276 | |
277 | $str = "Perl"; | |
278 | $str =~ /\x50/; # Match, "\x50" is "P". | |
279 | $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once. | |
280 | $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally. | |
281 | ||
282 | /\x{2603}\x{2602}/ # Snowman with an umbrella. | |
283 | # The Unicode character 2603 is a snowman, | |
284 | # the Unicode character 2602 is an umbrella. | |
285 | /\x{263B}/ # Black smiling face. | |
286 | /\x{263b}/ # Same, the hex digits A - F are case insensitive. | |
287 | ||
288 | =head2 Modifiers | |
289 | ||
290 | A number of backslash sequences have to do with changing the character, | |
291 | or characters following them. C<\l> will lowercase the character following | |
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292 | it, while C<\u> will uppercase (or, more accurately, titlecase) the |
293 | character following it. (They perform similar functionality as the | |
294 | functions C<lcfirst> and C<ucfirst>). | |
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295 | |
296 | To uppercase or lowercase several characters, one might want to use | |
297 | C<\L> or C<\U>, which will lowercase/uppercase all characters following | |
e2cb52ee | 298 | them, until either the end of the pattern, or the next occurrence of |
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299 | C<\E>, whatever comes first. They perform similar functionality as the |
300 | functions C<lc> and C<uc> do. | |
301 | ||
302 | C<\Q> is used to escape all characters following, up to the next C<\E> | |
303 | or the end of the pattern. C<\Q> adds a backslash to any character that | |
304 | isn't a letter, digit or underscore. This will ensure that any character | |
305 | between C<\Q> and C<\E> is matched literally, and will not be interpreted | |
306 | by the regexp engine. | |
307 | ||
308 | Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd. | |
309 | ||
310 | =head4 Examples | |
311 | ||
312 | $sid = "sid"; | |
313 | $greg = "GrEg"; | |
314 | $miranda = "(Miranda)"; | |
315 | $str =~ /\u$sid/; # Matches 'Sid' | |
316 | $str =~ /\L$greg/; # Matches 'greg' | |
317 | $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern | |
318 | # had been written as /\(Miranda\)/ | |
319 | ||
320 | =head2 Character classes | |
321 | ||
322 | Perl regular expressions have a large range of character classes. Some of | |
323 | the character classes are written as a backslash sequence. We will briefly | |
324 | discuss those here; full details of character classes can be found in | |
325 | L<perlrecharclass>. | |
326 | ||
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327 | C<\w> is a character class that matches any single I<word> character (letters, |
328 | digits, underscore). C<\d> is a character class that matches any decimal digit, | |
418e7b04 | 329 | while the character class C<\s> matches any whitespace character. |
99d59c4d | 330 | New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal |
418e7b04 | 331 | and vertical whitespace characters. |
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332 | |
333 | The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are | |
334 | character classes that match any character that isn't a word character, | |
418e7b04 | 335 | digit, whitespace, horizontal whitespace nor vertical whitespace. |
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336 | |
337 | Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical. | |
338 | ||
339 | =head3 Unicode classes | |
340 | ||
341 | C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to | |
342 | match a character that matches the given Unicode property; properties | |
343 | include things like "letter", or "thai character". Capitalizing the | |
344 | sequence to C<\PP> and C<\P{Property}> make the sequence match a character | |
345 | that doesn't match the given Unicode property. For more details, see | |
4948b50f | 346 | L<perlrecharclass/Backslash sequences> and |
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347 | L<perlunicode/Unicode Character Properties>. |
348 | ||
349 | Mnemonic: I<p>roperty. | |
350 | ||
351 | ||
352 | =head2 Referencing | |
353 | ||
354 | If capturing parenthesis are used in a regular expression, we can refer | |
355 | to the part of the source string that was matched, and match exactly the | |
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356 | same thing. There are three ways of referring to such I<backreference>: |
357 | absolutely, relatively, and by name. | |
358 | ||
359 | =for later add link to perlrecapture | |
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360 | |
361 | =head3 Absolute referencing | |
362 | ||
363 | A backslash sequence that starts with a backslash and is followed by a | |
364 | number is an absolute reference (but be aware of the caveat mentioned above). | |
df225385 | 365 | If the number is I<N>, it refers to the Nth set of parentheses - whatever |
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366 | has been matched by that set of parenthesis has to be matched by the C<\N> |
367 | as well. | |
368 | ||
369 | =head4 Examples | |
370 | ||
371 | /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat"). | |
372 | /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA"). | |
373 | ||
374 | ||
375 | =head3 Relative referencing | |
376 | ||
99d59c4d | 377 | New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>. |
8a118206 RGS |
378 | C<\g> takes a number as argument, with the number in curly braces (the |
379 | braces are optional). If the number (N) does not have a sign, it's a reference | |
380 | to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that | |
381 | C<\g> always refers to a capture group and will never be seen as an octal | |
e2cb52ee | 382 | escape). If the number is negative, the reference is relative, referring to |
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383 | the Nth group before the C<\g{-N}>. |
384 | ||
385 | The big advantage of C<\g{-N}> is that it makes it much easier to write | |
386 | patterns with references that can be interpolated in larger patterns, | |
387 | even if the larger pattern also contains capture groups. | |
388 | ||
389 | Mnemonic: I<g>roup. | |
390 | ||
391 | =head4 Examples | |
392 | ||
393 | /(A) # Buffer 1 | |
394 | ( # Buffer 2 | |
395 | (B) # Buffer 3 | |
396 | \g{-1} # Refers to buffer 3 (B) | |
397 | \g{-3} # Refers to buffer 1 (A) | |
398 | ) | |
399 | /x; # Matches "ABBA". | |
400 | ||
401 | my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc. | |
402 | /$qr$qr/ # Matches 'ababcdcd'. | |
403 | ||
404 | =head3 Named referencing | |
405 | ||
99d59c4d | 406 | Also new in perl 5.10.0 is the use of named capture buffers, which can be |
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407 | referred to by name. This is done with C<\g{name}>, which is a |
408 | backreference to the capture buffer with the name I<name>. | |
409 | ||
410 | To be compatible with .Net regular expressions, C<\g{name}> may also be | |
411 | written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. | |
412 | ||
413 | Note that C<\g{}> has the potential to be ambiguous, as it could be a named | |
414 | reference, or an absolute or relative reference (if its argument is numeric). | |
df225385 | 415 | However, names are not allowed to start with digits, nor are they allowed to |
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416 | contain a hyphen, so there is no ambiguity. |
417 | ||
418 | =head4 Examples | |
419 | ||
420 | /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat") | |
421 | /(?<word>\w+) \k{word}/ # Same. | |
422 | /(?<word>\w+) \k<word>/ # Same. | |
423 | /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/ | |
424 | # Match a four letter palindrome (e.g. "ABBA") | |
425 | ||
426 | =head2 Assertions | |
427 | ||
ac036724 | 428 | Assertions are conditions that have to be true; they don't actually |
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429 | match parts of the substring. There are six assertions that are written as |
430 | backslash sequences. | |
431 | ||
432 | =over 4 | |
433 | ||
434 | =item \A | |
435 | ||
436 | C<\A> only matches at the beginning of the string. If the C</m> modifier | |
437 | isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m> | |
438 | modifier is used, then C</^/> matches internal newlines, but the meaning | |
439 | of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning | |
440 | of the string regardless whether the C</m> modifier is used. | |
441 | ||
442 | =item \z, \Z | |
443 | ||
444 | C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't | |
445 | used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the | |
446 | end of the string, or before the newline at the end of the string. If the | |
447 | C</m> modifier is used, then C</$/> matches at internal newlines, but the | |
448 | meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at | |
449 | the end of the string (or just before a trailing newline) regardless whether | |
450 | the C</m> modifier is used. | |
451 | ||
452 | C<\z> is just like C<\Z>, except that it will not match before a trailing | |
453 | newline. C<\z> will only match at the end of the string - regardless of the | |
454 | modifiers used, and not before a newline. | |
455 | ||
456 | =item \G | |
457 | ||
458 | C<\G> is usually only used in combination with the C</g> modifier. If the | |
459 | C</g> modifier is used (and the match is done in scalar context), Perl will | |
460 | remember where in the source string the last match ended, and the next time, | |
461 | it will start the match from where it ended the previous time. | |
462 | ||
463 | C<\G> matches the point where the previous match ended, or the beginning | |
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464 | of the string if there was no previous match. |
465 | ||
466 | =for later add link to perlremodifiers | |
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467 | |
468 | Mnemonic: I<G>lobal. | |
469 | ||
470 | =item \b, \B | |
471 | ||
472 | C<\b> matches at any place between a word and a non-word character; C<\B> | |
473 | matches at any place between characters where C<\b> doesn't match. C<\b> | |
474 | and C<\B> assume there's a non-word character before the beginning and after | |
475 | the end of the source string; so C<\b> will match at the beginning (or end) | |
476 | of the source string if the source string begins (or ends) with a word | |
477 | character. Otherwise, C<\B> will match. | |
478 | ||
479 | Mnemonic: I<b>oundary. | |
480 | ||
481 | =back | |
482 | ||
483 | =head4 Examples | |
484 | ||
485 | "cat" =~ /\Acat/; # Match. | |
486 | "cat" =~ /cat\Z/; # Match. | |
487 | "cat\n" =~ /cat\Z/; # Match. | |
488 | "cat\n" =~ /cat\z/; # No match. | |
489 | ||
490 | "cat" =~ /\bcat\b/; # Matches. | |
491 | "cats" =~ /\bcat\b/; # No match. | |
492 | "cat" =~ /\bcat\B/; # No match. | |
493 | "cats" =~ /\bcat\B/; # Match. | |
494 | ||
495 | while ("cat dog" =~ /(\w+)/g) { | |
496 | print $1; # Prints 'catdog' | |
497 | } | |
498 | while ("cat dog" =~ /\G(\w+)/g) { | |
499 | print $1; # Prints 'cat' | |
500 | } | |
501 | ||
502 | =head2 Misc | |
503 | ||
504 | Here we document the backslash sequences that don't fall in one of the | |
505 | categories above. They are: | |
506 | ||
507 | =over 4 | |
508 | ||
509 | =item \C | |
510 | ||
511 | C<\C> always matches a single octet, even if the source string is encoded | |
512 | in UTF-8 format, and the character to be matched is a multi-octet character. | |
513 | C<\C> was introduced in perl 5.6. | |
514 | ||
515 | Mnemonic: oI<C>tet. | |
516 | ||
517 | =item \K | |
518 | ||
99d59c4d | 519 | This is new in perl 5.10.0. Anything that is matched left of C<\K> is |
8a118206 RGS |
520 | not included in C<$&> - and will not be replaced if the pattern is |
521 | used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x> | |
522 | instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>. | |
523 | ||
524 | Mnemonic: I<K>eep. | |
525 | ||
df225385 KW |
526 | =item \N |
527 | ||
b3b85878 KW |
528 | This is a new experimental feature in perl 5.12.0. It matches any character |
529 | that is not a newline. It is a short-hand for writing C<[^\n]>, and is | |
530 | identical to the C<.> metasymbol, except under the C</s> flag, which changes | |
531 | the meaning of C<.>, but not C<\N>. | |
df225385 | 532 | |
e526e8bb KW |
533 | Note that C<\N{...}> can mean a |
534 | L<named or numbered character|/Named or numbered characters>. | |
df225385 KW |
535 | |
536 | Mnemonic: Complement of I<\n>. | |
537 | ||
8a118206 | 538 | =item \R |
6b46370c | 539 | X<\R> |
8a118206 RGS |
540 | |
541 | C<\R> matches a I<generic newline>, that is, anything that is considered | |
542 | a newline by Unicode. This includes all characters matched by C<\v> | |
418e7b04 | 543 | (vertical whitespace), and the multi character sequence C<"\x0D\x0A"> |
8a118206 | 544 | (carriage return followed by a line feed, aka the network newline, or |
58151fe4 KW |
545 | the newline used in Windows text files). C<\R> is equivalent to |
546 | C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one | |
547 | character, it cannot be put inside a bracketed character class; C</[\R]/> is an | |
548 | error; use C<\v> instead. C<\R> was introduced in perl 5.10.0. | |
8a118206 | 549 | |
10fdd326 JH |
550 | Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>, |
551 | and more importantly because Unicode recommends such a regular expression | |
552 | metacharacter, and suggests C<\R> as the notation. | |
8a118206 RGS |
553 | |
554 | =item \X | |
6b46370c | 555 | X<\X> |
8a118206 | 556 | |
0111a78f | 557 | This matches a Unicode I<extended grapheme cluster>. |
8a118206 | 558 | |
10fdd326 | 559 | C<\X> matches quite well what normal (non-Unicode-programmer) usage |
0111a78f | 560 | would consider a single character. As an example, consider a G with some sort |
c670e63a | 561 | of diacritic mark, such as an arrow. There is no such single character in |
df225385 | 562 | Unicode, but one can be composed by using a G followed by a Unicode "COMBINING |
c670e63a KW |
563 | UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it |
564 | were a single character. | |
10fdd326 | 565 | |
8a118206 RGS |
566 | Mnemonic: eI<X>tended Unicode character. |
567 | ||
568 | =back | |
569 | ||
570 | =head4 Examples | |
571 | ||
572 | "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8. | |
573 | ||
574 | $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'. | |
575 | $str =~ s/(.)\K\1//g; # Delete duplicated characters. | |
576 | ||
577 | "\n" =~ /^\R$/; # Match, \n is a generic newline. | |
578 | "\r" =~ /^\R$/; # Match, \r is a generic newline. | |
579 | "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline. | |
580 | ||
581 | "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above. | |
582 | ||
583 | =cut |