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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 | |
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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]. |
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37 | |
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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 | |
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65 | \000 Octal escape sequence. |
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66 | \1 Absolute backreference. Not in []. |
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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 []. |
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71 | \cX Control-X |
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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. |
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76 | \E Turn off \Q, \L and \U processing. Not in []. |
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77 | \f Form feed. |
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78 | \g{}, \g1 Named, absolute or relative backreference. Not in [] |
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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 []. |
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86 | \n (Logical) newline character. |
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87 | \N Any character but newline. Experimental. Not in []. |
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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. |
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91 | \Q Quotemeta till \E. Not in []. |
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92 | \r Return character. |
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93 | \R Generic new line. Not in []. |
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94 | \s Character class for whitespace. |
95 | \S Character class for non whitespace. |
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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 | |
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112 | A handful of characters have a dedicated I<character escape>. The following |
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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>.) |
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116 | |
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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 | |
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167 | =head3 Named or numbered characters |
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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>. |
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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". |
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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 | |
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221 | =head4 Examples (assuming an ASCII platform) |
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222 | |
223 | $str = "Perl"; |
224 | $str =~ /\120/; # Match, "\120" is "P". |
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225 | $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once |
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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 | |
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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 | |
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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 | |
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275 | =head4 Examples (assuming an ASCII platform) |
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276 | |
277 | $str = "Perl"; |
278 | $str =~ /\x50/; # Match, "\x50" is "P". |
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279 | $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once |
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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 |
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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, |
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329 | while the character class C<\s> matches any whitespace character. |
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330 | New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal |
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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, |
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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 |
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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). |
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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 | |
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377 | New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>. |
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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 |
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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 | |
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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). |
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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 | |
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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 $1; # Prints 'cat' |
500 | } |
501 | |
502 | =head2 Misc |
503 | |
504 | Here we document the backslash sequences that don't fall in one of the |
505 | categories above. They are: |
506 | |
507 | =over 4 |
508 | |
509 | =item \C |
510 | |
511 | C<\C> always matches a single octet, even if the source string is encoded |
512 | in UTF-8 format, and the character to be matched is a multi-octet character. |
513 | C<\C> was introduced in perl 5.6. |
514 | |
515 | Mnemonic: oI<C>tet. |
516 | |
517 | =item \K |
518 | |
99d59c4d |
519 | This is new in perl 5.10.0. Anything that is matched left of C<\K> is |
8a118206 |
520 | not included in C<$&> - 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 |
526 | =item \N |
527 | |
b3b85878 |
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 |
533 | Note that C<\N{...}> can mean a |
534 | L<named or numbered character|/Named or numbered characters>. |
df225385 |
535 | |
536 | Mnemonic: Complement of I<\n>. |
537 | |
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538 | =item \R |
6b46370c |
539 | X<\R> |
8a118206 |
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 |
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. |
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549 | |
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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. |
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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 |
563 | UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it |
564 | were a single character. |
10fdd326 |
565 | |
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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 | |
f822d0dd |
574 | $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz' |
8a118206 |
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 |