1PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1)
2
6 perlrebackslash - Perl Regular Expression Backslash Sequences and
7 Escapes
8
10 The top level documentation about Perl regular expressions is found in
11 perlre.
12 This document describes all backslash and escape sequences. After
14 explaining the role of the backslash, it lists all the sequences that
15 have a special meaning in Perl regular expressions (in alphabetical
16 order), then describes each of them.
17 Most sequences are described in detail in different documents; the
19 primary purpose of this document is to have a quick reference guide
20 describing all backslash and escape sequences.
21 The backslash
23 In a regular expression, the backslash can perform one of two tasks: it
24 either takes away the special meaning of the character following it
25 (for instance, "\|" matches a vertical bar, it's not an alternation),
26 or it is the start of a backslash or escape sequence.
27 The rules determining what it is are quite simple: if the character
29 following the backslash is an ASCII punctuation (non-word) character
30 (that is, anything that is not a letter, digit, or underscore), then
31 the backslash just takes away any special meaning of the character
32 following it.
33 If the character following the backslash is an ASCII letter or an ASCII
35 digit, then the sequence may be special; if so, it's listed below. A
36 few letters have not been used yet, so escaping them with a backslash
37 doesn't change them to be special. A future version of Perl may assign
38 a special meaning to them, so if you have warnings turned on, Perl
39 issues a warning if you use such a sequence. [1].
40 It is however guaranteed that backslash or escape sequences never have
42 a punctuation character following the backslash, not now, and not in a
43 future version of Perl 5. So it is safe to put a backslash in front of
44 a non-word character.
45 Note that the backslash itself is special; if you want to match a
47 backslash, you have to escape the backslash with a backslash: "/\\/"
48 matches a single backslash.
49 [1] There is one exception. If you use an alphanumeric character as the
51 delimiter of your pattern (which you probably shouldn't do for
52 readability reasons), you have to escape the delimiter if you want
53 to match it. Perl won't warn then. See also "Gory details of
54 parsing quoted constructs" in perlop.
55 All the sequences and escapes
57 Those not usable within a bracketed character class (like "[\da-z]")
58 are marked as "Not in []."
59 \000 Octal escape sequence. See also \o{}.
61 \1 Absolute backreference. Not in [].
62 \a Alarm or bell.
63 \A Beginning of string. Not in [].
64 \b{}, \b Boundary. (\b is a backspace in []).
65 \B{}, \B Not a boundary. Not in [].
66 \cX Control-X.
67 \d Match any digit character.
68 \D Match any character that isn't a digit.
69 \e Escape character.
70 \E Turn off \Q, \L and \U processing. Not in [].
71 \f Form feed.
72 \F Foldcase till \E. Not in [].
73 \g{}, \g1 Named, absolute or relative backreference.
74 Not in [].
75 \G Pos assertion. Not in [].
76 \h Match any horizontal whitespace character.
77 \H Match any character that isn't horizontal whitespace.
78 \k{}, \k<>, \k'' Named backreference. Not in [].
79 \K Keep the stuff left of \K. Not in [].
80 \l Lowercase next character. Not in [].
81 \L Lowercase till \E. Not in [].
82 \n (Logical) newline character.
83 \N Match any character but newline. Not in [].
84 \N{} Named or numbered (Unicode) character or sequence.
85 \o{} Octal escape sequence.
86 \p{}, \pP Match any character with the given Unicode property.
87 \P{}, \PP Match any character without the given property.
88 \Q Quote (disable) pattern metacharacters till \E. Not
89 in [].
90 \r Return character.
91 \R Generic new line. Not in [].
92 \s Match any whitespace character.
93 \S Match any character that isn't a whitespace.
94 \t Tab character.
95 \u Titlecase next character. Not in [].
96 \U Uppercase till \E. Not in [].
97 \v Match any vertical whitespace character.
98 \V Match any character that isn't vertical whitespace
99 \w Match any word character.
100 \W Match any character that isn't a word character.
101 \x{}, \x00 Hexadecimal escape sequence.
102 \X Unicode "extended grapheme cluster". Not in [].
103 \z End of string. Not in [].
104 \Z End of string. Not in [].
105 Character Escapes
107 Fixed characters
108 A handful of characters have a dedicated character escape. The
110 following table shows them, along with their ASCII code points (in
111 decimal and hex), their ASCII name, the control escape on ASCII
112 platforms and a short description. (For EBCDIC platforms, see
113 "OPERATOR DIFFERENCES" in perlebcdic.)
114 Seq. Code Point ASCII Cntrl 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 [1] "\b" is the backspace character only inside a character class.
126 Outside a character class, "\b" alone is a
127 word-character/non-word-character boundary, and "\b{}" is some
128 other type of boundary.
129 [2] "\n" matches a logical newline. Perl converts between "\n" and your
131 OS's native newline character when reading from or writing to text
132 files.
133 Example
135 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
137 Control characters
139 "\c" is used to denote a control character; the character following
141 "\c" determines the value of the construct. For example the value of
142 "\cA" is chr(1), and the value of "\cb" is chr(2), etc. The gory
143 details are in "Regexp Quote-Like Operators" in perlop. A complete
144 list of what chr(1), etc. means for ASCII and EBCDIC platforms is in
145 "OPERATOR DIFFERENCES" in perlebcdic.
146 Note that "\c\" alone at the end of a regular expression (or doubled-
148 quoted string) is not valid. The backslash must be followed by another
149 character. That is, "\c\X" means "chr(28) . 'X'" for all characters X.
150 To write platform-independent code, you must use "\N{NAME}" instead,
152 like "\N{ESCAPE}" or "\N{U+001B}", see charnames.
153 Mnemonic: control character.
155 Example
157 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
159 Named or numbered characters and character sequences
161 Unicode characters have a Unicode name and numeric code point (ordinal)
163 value. Use the "\N{}" construct to specify a character by either of
164 these values. Certain sequences of characters also have names.
165 To specify by name, the name of the character or character sequence
167 goes between the curly braces.
168 To specify a character by Unicode code point, use the form "\N{U+code
170 point}", where code point is a number in hexadecimal that gives the
171 code point that Unicode has assigned to the desired character. It is
172 customary but not required to use leading zeros to pad the number to 4
173 digits. Thus "\N{U+0041}" means "LATIN CAPITAL LETTER A", and you will
174 rarely see it written without the two leading zeros. "\N{U+0041}"
175 means "A" even on EBCDIC machines (where the ordinal value of "A" is
176 not 0x41).
177 Blanks may freely be inserted adjacent to but within the braces
179 enclosing the name or code point. So "\N{ U+0041 }" is perfectly
180 legal.
181 It is even possible to give your own names to characters and character
183 sequences by using the charnames module. These custom names are
184 lexically scoped, and so a given code point may have different names in
185 different scopes. The name used is what is in effect at the time the
186 "\N{}" is expanded. For patterns in double-quotish context, that means
187 at the time the pattern is parsed. But for patterns that are
188 delimitted by single quotes, the expansion is deferred until pattern
189 compilation time, which may very well have a different "charnames"
190 translator in effect.
191 (There is an expanded internal form that you may see in debug output:
193 "\N{U+code point.code point...}". The "..." means any number of these
194 code points separated by dots. This represents the sequence formed by
195 the characters. This is an internal form only, subject to change, and
196 you should not try to use it yourself.)
197 Mnemonic: Named character.
199 Note that a character or character sequence expressed as a named or
201 numbered character is considered a character without special meaning by
202 the regex engine, and will match "as is".
203 Example
205 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
207 use charnames 'Cyrillic'; # Loads Cyrillic names.
209 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
210 Octal escapes
212 There are two forms of octal escapes. Each is used to specify a
214 character by its code point specified in base 8.
215 One form, available starting in Perl 5.14 looks like "\o{...}", where
217 the dots represent one or more octal digits. It can be used for any
218 Unicode character.
219 It was introduced to avoid the potential problems with the other form,
221 available in all Perls. That form consists of a backslash followed by
222 three octal digits. One problem with this form is that it can look
223 exactly like an old-style backreference (see "Disambiguation rules
224 between old-style octal escapes and backreferences" below.) You can
225 avoid this by making the first of the three digits always a zero, but
226 that makes \077 the largest code point specifiable.
227 In some contexts, a backslash followed by two or even one octal digits
229 may be interpreted as an octal escape, sometimes with a warning, and
230 because of some bugs, sometimes with surprising results. Also, if you
231 are creating a regex out of smaller snippets concatenated together, and
232 you use fewer than three digits, the beginning of one snippet may be
233 interpreted as adding digits to the ending of the snippet before it.
234 See "Absolute referencing" for more discussion and examples of the
235 snippet problem.
236 Note that a character expressed as an octal escape is considered a
238 character without special meaning by the regex engine, and will match
239 "as is".
240 To summarize, the "\o{}" form is always safe to use, and the other form
242 is safe to use for code points through \077 when you use exactly three
243 digits to specify them.
244 Mnemonic: 0ctal or octal.
246 Examples (assuming an ASCII platform)
248 $str = "Perl";
250 $str =~ /\o{120}/; # Match, "\120" is "P".
251 $str =~ /\120/; # Same.
252 $str =~ /\o{120}+/; # Match, "\120" is "P",
253 # it's repeated at least once.
254 $str =~ /\120+/; # Same.
255 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
256 /\o{23073}/ # Black foreground, white background smiling face.
257 /\o{4801234567}/ # Raises a warning, and yields chr(4).
258 /\o{ 400}/ # LATIN CAPITAL LETTER A WITH MACRON
259 /\o{ 400 }/ # Same. These show blanks are allowed adjacent to
260 # the braces
261 Disambiguation rules between old-style octal escapes and backreferences
263 Octal escapes of the "\000" form outside of bracketed character classes
265 potentially clash with old-style backreferences (see "Absolute
266 referencing" below). They both consist of a backslash followed by
267 numbers. So Perl has to use heuristics to determine whether it is a
268 backreference or an octal escape. Perl uses the following rules to
269 disambiguate:
270 1. If the backslash is followed by a single digit, it's a
272 backreference.
273 2. If the first digit following the backslash is a 0, it's an octal
275 escape.
276 3. If the number following the backslash is N (in decimal), and Perl
278 already has seen N capture groups, Perl considers this a
279 backreference. Otherwise, it considers it an octal escape. If N
280 has more than three digits, Perl takes only the first three for the
281 octal escape; the rest are matched as is.
282 my $pat = "(" x 999;
284 $pat .= "a";
285 $pat .= ")" x 999;
286 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
287 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
288 # and \1000 is seen as \100 (a '@') and a '0'.
289 You can force a backreference interpretation always by using the
291 "\g{...}" form. You can the force an octal interpretation always by
292 using the "\o{...}" form, or for numbers up through \077 (= 63
293 decimal), by using three digits, beginning with a "0".
294 Hexadecimal escapes
296 Like octal escapes, there are two forms of hexadecimal escapes, but
298 both start with the sequence "\x". This is followed by either exactly
299 two hexadecimal digits forming a number, or a hexadecimal number of
300 arbitrary length surrounded by curly braces. The hexadecimal number is
301 the code point of the character you want to express.
302 Note that a character expressed as one of these escapes is considered a
304 character without special meaning by the regex engine, and will match
305 "as is".
306 Mnemonic: hexadecimal.
308 Examples (assuming an ASCII platform)
310 $str = "Perl";
312 $str =~ /\x50/; # Match, "\x50" is "P".
313 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
314 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
315 /\x{2603}\x{2602}/ # Snowman with an umbrella.
317 # The Unicode character 2603 is a snowman,
318 # the Unicode character 2602 is an umbrella.
319 /\x{263B}/ # Black smiling face.
320 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
321 /\x{ 263b }/ # Same, showing optional blanks adjacent to the
322 # braces
323 Modifiers
325 A number of backslash sequences have to do with changing the character,
326 or characters following them. "\l" will lowercase the character
327 following it, while "\u" will uppercase (or, more accurately,
328 titlecase) the character following it. They provide functionality
329 similar to the functions "lcfirst" and "ucfirst".
330 To uppercase or lowercase several characters, one might want to use
332 "\L" or "\U", which will lowercase/uppercase all characters following
333 them, until either the end of the pattern or the next occurrence of
334 "\E", whichever comes first. They provide functionality similar to what
335 the functions "lc" and "uc" provide.
336 "\Q" is used to quote (disable) pattern metacharacters, up to the next
338 "\E" or the end of the pattern. "\Q" adds a backslash to any character
339 that could have special meaning to Perl. In the ASCII range, it quotes
340 every character that isn't a letter, digit, or underscore. See
341 "quotemeta" in perlfunc for details on what gets quoted for non-ASCII
342 code points. Using this ensures that any character between "\Q" and
343 "\E" will be matched literally, not interpreted as a metacharacter by
344 the regex engine.
345 "\F" can be used to casefold all characters following, up to the next
347 "\E" or the end of the pattern. It provides the functionality similar
348 to the "fc" function.
349 Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.
351 Examples
353 $sid = "sid";
355 $greg = "GrEg";
356 $miranda = "(Miranda)";
357 $str =~ /\u$sid/; # Matches 'Sid'
358 $str =~ /\L$greg/; # Matches 'greg'
359 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
360 # had been written as /\(Miranda\)/
361 Character classes
363 Perl regular expressions have a large range of character classes. Some
364 of the character classes are written as a backslash sequence. We will
365 briefly discuss those here; full details of character classes can be
366 found in perlrecharclass.
367 "\w" is a character class that matches any single word character
369 (letters, digits, Unicode marks, and connector punctuation (like the
370 underscore)). "\d" is a character class that matches any decimal
371 digit, while the character class "\s" matches any whitespace character.
372 New in perl 5.10.0 are the classes "\h" and "\v" which match horizontal
373 and vertical whitespace characters.
374 The exact set of characters matched by "\d", "\s", and "\w" varies
376 depending on various pragma and regular expression modifiers. It is
377 possible to restrict the match to the ASCII range by using the "/a"
378 regular expression modifier. See perlrecharclass.
379 The uppercase variants ("\W", "\D", "\S", "\H", and "\V") are character
381 classes that match, respectively, any character that isn't a word
382 character, digit, whitespace, horizontal whitespace, or vertical
383 whitespace.
384 Mnemonics: word, digit, space, horizontal, vertical.
386 Unicode classes
388 "\pP" (where "P" is a single letter) and "\p{Property}" are used to
390 match a character that matches the given Unicode property; properties
391 include things like "letter", or "thai character". Capitalizing the
392 sequence to "\PP" and "\P{Property}" make the sequence match a
393 character that doesn't match the given Unicode property. For more
394 details, see "Backslash sequences" in perlrecharclass and "Unicode
395 Character Properties" in perlunicode.
396 Mnemonic: property.
398 Referencing
400 If capturing parenthesis are used in a regular expression, we can refer
401 to the part of the source string that was matched, and match exactly
402 the same thing. There are three ways of referring to such
403 backreference: absolutely, relatively, and by name.
404 Absolute referencing
406 Either "\gN" (starting in Perl 5.10.0), or "\N" (old-style) where N is
408 a positive (unsigned) decimal number of any length is an absolute
409 reference to a capturing group.
410 N refers to the Nth set of parentheses, so "\gN" refers to whatever has
412 been matched by that set of parentheses. Thus "\g1" refers to the
413 first capture group in the regex.
414 The "\gN" form can be equivalently written as "\g{N}" which avoids
416 ambiguity when building a regex by concatenating shorter strings.
417 Otherwise if you had a regex "qr/$a$b/", and $a contained "\g1", and $b
418 contained "37", you would get "/\g137/" which is probably not what you
419 intended.
420 In the "\N" form, N must not begin with a "0", and there must be at
422 least N capturing groups, or else N is considered an octal escape (but
423 something like "\18" is the same as "\0018"; that is, the octal escape
424 "\001" followed by a literal digit "8").
425 Mnemonic: group.
427 Examples
429 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
431 /(\w+) \1/; # Same thing; written old-style.
432 /(\w+) \g{1}/; # Same, using the safer braced notation
433 /(\w+) \g{ 1 }/;# Same, showing optional blanks adjacent to the braces
434 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
435 Relative referencing
437 "\g-N" (starting in Perl 5.10.0) is used for relative addressing. (It
439 can be written as "\g{-N}".) It refers to the Nth group before the
440 "\g{-N}".
441 The big advantage of this form is that it makes it much easier to write
443 patterns with references that can be interpolated in larger patterns,
444 even if the larger pattern also contains capture groups.
445 Examples
447 /(A) # Group 1
449 ( # Group 2
450 (B) # Group 3
451 \g{-1} # Refers to group 3 (B)
452 \g{-3} # Refers to group 1 (A)
453 \g{ -3 } # Same, showing optional blanks adjacent to the braces
454 )
455 /x; # Matches "ABBA".
456 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
458 /$qr$qr/ # Matches 'ababcdcd'.
459 Named referencing
461 "\g{name}" (starting in Perl 5.10.0) can be used to back refer to a
463 named capture group, dispensing completely with having to think about
464 capture buffer positions.
465 To be compatible with .Net regular expressions, "\g{name}" may also be
467 written as "\k{name}", "\k<name>" or "\k'name'".
468 To prevent any ambiguity, name must not start with a digit nor contain
470 a hyphen.
471 Examples
473 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
475 /(?<word>\w+) \k{word}/ # Same.
476 /(?<word>\w+) \g{ word }/ # Same, showing optional blanks adjacent to
477 # the braces
478 /(?<word>\w+) \k{ word }/ # Same.
479 /(?<word>\w+) \k<word>/ # Same. There are no braces, so no blanks
480 # are permitted
481 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
482 # Match a four letter palindrome (e.g.
483 # "ABBA")
484 Assertions
486 Assertions are conditions that have to be true; they don't actually
487 match parts of the substring. There are six assertions that are written
488 as backslash sequences.
489 \A "\A" only matches at the beginning of the string. If the "/m"
491 modifier isn't used, then "/\A/" is equivalent to "/^/". However,
492 if the "/m" modifier is used, then "/^/" matches internal newlines,
493 but the meaning of "/\A/" isn't changed by the "/m" modifier. "\A"
494 matches at the beginning of the string regardless whether the "/m"
495 modifier is used.
496 \z, \Z
498 "\z" and "\Z" match at the end of the string. If the "/m" modifier
499 isn't used, then "/\Z/" is equivalent to "/$/"; that is, it matches
500 at the end of the string, or one before the newline at the end of
501 the string. If the "/m" modifier is used, then "/$/" matches at
502 internal newlines, but the meaning of "/\Z/" isn't changed by the
503 "/m" modifier. "\Z" matches at the end of the string (or just
504 before a trailing newline) regardless whether the "/m" modifier is
505 used.
506 "\z" is just like "\Z", except that it does not match before a
508 trailing newline. "\z" matches at the end of the string only,
509 regardless of the modifiers used, and not just before a newline.
510 It is how to anchor the match to the true end of the string under
511 all conditions.
512 \G "\G" is usually used only in combination with the "/g" modifier. If
514 the "/g" modifier is used and the match is done in scalar context,
515 Perl remembers where in the source string the last match ended, and
516 the next time, it will start the match from where it ended the
517 previous time.
518 "\G" matches the point where the previous match on that string
520 ended, or the beginning of that string if there was no previous
521 match.
522 Mnemonic: Global.
524 \b{}, \b, \B{}, \B
526 "\b{...}", available starting in v5.22, matches a boundary (between
527 two characters, or before the first character of the string, or
528 after the final character of the string) based on the Unicode rules
529 for the boundary type specified inside the braces. The boundary
530 types are given a few paragraphs below. "\B{...}" matches at any
531 place between characters where "\b{...}" of the same type doesn't
532 match.
533 "\b" when not immediately followed by a "{" is available in all
535 Perls. It matches at any place between a word (something matched
536 by "\w") and a non-word character ("\W"); "\B" when not immediately
537 followed by a "{" matches at any place between characters where
538 "\b" doesn't match. To get better word matching of natural
539 language text, see "\b{wb}" below.
540 "\b" and "\B" assume there's a non-word character before the
542 beginning and after the end of the source string; so "\b" will
543 match at the beginning (or end) of the source string if the source
544 string begins (or ends) with a word character. Otherwise, "\B" will
545 match.
546 Do not use something like "\b=head\d\b" and expect it to match the
548 beginning of a line. It can't, because for there to be a boundary
549 before the non-word "=", there must be a word character immediately
550 previous. All plain "\b" and "\B" boundary determinations look for
551 word characters alone, not for non-word characters nor for string
552 ends. It may help to understand how "\b" and "\B" work by equating
553 them as follows:
554 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
556 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
557 In contrast, "\b{...}" and "\B{...}" may or may not match at the
559 beginning and end of the line, depending on the boundary type.
560 These implement the Unicode default boundaries, specified in
561 <https://www.unicode.org/reports/tr14/> and
562 <https://www.unicode.org/reports/tr29/>. The boundary types are:
563 "\b{gcb}" or "\b{g}"
565 This matches a Unicode "Grapheme Cluster Boundary". (Actually
566 Perl always uses the improved "extended" grapheme cluster").
567 These are explained below under "\X". In fact, "\X" is another
568 way to get the same functionality. It is equivalent to
569 "/.+?\b{gcb}/". Use whichever is most convenient for your
570 situation.
571 "\b{lb}"
573 This matches according to the default Unicode Line Breaking
574 Algorithm (<https://www.unicode.org/reports/tr14/>), as
575 customized in that document (Example 7 of revision 35
576 <https://www.unicode.org/reports/tr14/tr14-35.html#Example7>)
577 for better handling of numeric expressions.
578 This is suitable for many purposes, but the Unicode::LineBreak
580 module is available on CPAN that provides many more features,
581 including customization.
582 "\b{sb}"
584 This matches a Unicode "Sentence Boundary". This is an aid to
585 parsing natural language sentences. It gives good, but
586 imperfect results. For example, it thinks that "Mr. Smith" is
587 two sentences. More details are at
588 <https://www.unicode.org/reports/tr29/>. Note also that it
589 thinks that anything matching "\R" (except form feed and
590 vertical tab) is a sentence boundary. "\b{sb}" works with text
591 designed for word-processors which wrap lines automatically for
592 display, but hard-coded line boundaries are considered to be
593 essentially the ends of text blocks (paragraphs really), and
594 hence the ends of sentences. "\b{sb}" doesn't do well with
595 text containing embedded newlines, like the source text of the
596 document you are reading. Such text needs to be preprocessed
597 to get rid of the line separators before looking for sentence
598 boundaries. Some people view this as a bug in the Unicode
599 standard, and this behavior is quite subject to change in
600 future Perl versions.
601 "\b{wb}"
603 This matches a Unicode "Word Boundary", but tailored to Perl
604 expectations. This gives better (though not perfect) results
605 for natural language processing than plain "\b" (without
606 braces) does. For example, it understands that apostrophes can
607 be in the middle of words and that parentheses aren't (see the
608 examples below). More details are at
609 <https://www.unicode.org/reports/tr29/>.
610 The current Unicode definition of a Word Boundary matches
612 between every white space character. Perl tailors this,
613 starting in version 5.24, to generally not break up spans of
614 white space, just as plain "\b" has always functioned. This
615 allows "\b{wb}" to be a drop-in replacement for "\b", but with
616 generally better results for natural language processing. (The
617 exception to this tailoring is when a span of white space is
618 immediately followed by something like U+0303, COMBINING TILDE.
619 If the final space character in the span is a horizontal white
620 space, it is broken out so that it attaches instead to the
621 combining character. To be precise, if a span of white space
622 that ends in a horizontal space has the character immediately
623 following it have any of the Word Boundary property values
624 "Extend", "Format" or "ZWJ", the boundary between the final
625 horizontal space character and the rest of the span matches
626 "\b{wb}". In all other cases the boundary between two white
627 space characters matches "\B{wb}".)
628 It is important to realize when you use these Unicode boundaries,
630 that you are taking a risk that a future version of Perl which
631 contains a later version of the Unicode Standard will not work
632 precisely the same way as it did when your code was written. These
633 rules are not considered stable and have been somewhat more subject
634 to change than the rest of the Standard. Unicode reserves the
635 right to change them at will, and Perl reserves the right to update
636 its implementation to Unicode's new rules. In the past, some
637 changes have been because new characters have been added to the
638 Standard which have different characteristics than all previous
639 characters, so new rules are formulated for handling them. These
640 should not cause any backward compatibility issues. But some
641 changes have changed the treatment of existing characters because
642 the Unicode Technical Committee has decided that the change is
643 warranted for whatever reason. This could be to fix a bug, or
644 because they think better results are obtained with the new rule.
645 It is also important to realize that these are default boundary
647 definitions, and that implementations may wish to tailor the
648 results for particular purposes and locales. For example, some
649 languages, such as Japanese and Thai, require dictionary lookup to
650 accurately determine word boundaries.
651 Mnemonic: boundary.
653 Examples
655 "cat" =~ /\Acat/; # Match.
657 "cat" =~ /cat\Z/; # Match.
658 "cat\n" =~ /cat\Z/; # Match.
659 "cat\n" =~ /cat\z/; # No match.
660 "cat" =~ /\bcat\b/; # Matches.
662 "cats" =~ /\bcat\b/; # No match.
663 "cat" =~ /\bcat\B/; # No match.
664 "cats" =~ /\bcat\B/; # Match.
665 while ("cat dog" =~ /(\w+)/g) {
667 print $1; # Prints 'catdog'
668 }
669 while ("cat dog" =~ /\G(\w+)/g) {
670 print $1; # Prints 'cat'
671 }
672 my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
674 print join("|", $s =~ m/ ( .+? \b ) /xg), "\n";
675 print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
676 prints
677 He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
678 He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"
679 Misc
681 Here we document the backslash sequences that don't fall in one of the
682 categories above. These are:
683 \K This appeared in perl 5.10.0. Anything matched left of "\K" is not
685 included in $&, and will not be replaced if the pattern is used in
686 a substitution. This lets you write "s/PAT1 \K PAT2/REPL/x" instead
687 of "s/(PAT1) PAT2/${1}REPL/x" or "s/(?<=PAT1) PAT2/REPL/x".
688 Mnemonic: Keep.
690 \N This feature, available starting in v5.12, matches any character
692 that is not a newline. It is a short-hand for writing "[^\n]", and
693 is identical to the "." metasymbol, except under the "/s" flag,
694 which changes the meaning of ".", but not "\N".
695 Note that "\N{...}" can mean a named or numbered character .
697 Mnemonic: Complement of \n.
699 \R "\R" matches a generic newline; that is, anything considered a
701 linebreak sequence by Unicode. This includes all characters matched
702 by "\v" (vertical whitespace), and the multi character sequence
703 "\x0D\x0A" (carriage return followed by a line feed, sometimes
704 called the network newline; it's the end of line sequence used in
705 Microsoft text files opened in binary mode). "\R" is equivalent to
706 "(?>\x0D\x0A|\v)". (The reason it doesn't backtrack is that the
707 sequence is considered inseparable. That means that
708 "\x0D\x0A" =~ /^\R\x0A$/ # No match
710 fails, because the "\R" matches the entire string, and won't
712 backtrack to match just the "\x0D".) Since "\R" can match a
713 sequence of more than one character, it cannot be put inside a
714 bracketed character class; "/[\R]/" is an error; use "\v" instead.
715 "\R" was introduced in perl 5.10.0.
716 Note that this does not respect any locale that might be in effect;
718 it matches according to the platform's native character set.
719 Mnemonic: none really. "\R" was picked because PCRE already uses
721 "\R", and more importantly because Unicode recommends such a
722 regular expression metacharacter, and suggests "\R" as its
723 notation.
724 \X This matches a Unicode extended grapheme cluster.
726 "\X" matches quite well what normal (non-Unicode-programmer) usage
728 would consider a single character. As an example, consider a G
729 with some sort of diacritic mark, such as an arrow. There is no
730 such single character in Unicode, but one can be composed by using
731 a G followed by a Unicode "COMBINING UPWARDS ARROW BELOW", and
732 would be displayed by Unicode-aware software as if it were a single
733 character.
734 The match is greedy and non-backtracking, so that the cluster is
736 never broken up into smaller components.
737 See also "\b{gcb}".
739 Mnemonic: eXtended Unicode character.
741 Examples
743 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
745 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
746 "\n" =~ /^\R$/; # Match, \n is a generic newline.
748 "\r" =~ /^\R$/; # Match, \r is a generic newline.
749 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
750 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
752perl v5.34.0 2021-10-18 PERLREBACKSLASH(1)