1PERLREBACKSLASH(1)     Perl Programmers Reference Guide     PERLREBACKSLASH(1)
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NAME

6       perlrebackslash - Perl Regular Expression Backslash Sequences and
7       Escapes
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DESCRIPTION

10       The top level documentation about Perl regular expressions is found in
11       perlre.
12
13       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
18       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
22   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
28       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
34       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
41       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
46       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
50       [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
56   All the sequences and escapes
57       Those not usable within a bracketed character class (like "[\da-z]")
58       are marked as "Not in []."
59
60        \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
106   Character Escapes
107       Fixed characters
108
109       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
115        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
125       [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
130       [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
134       Example
135
136        $str =~ /\t/;   # Matches if $str contains a (horizontal) tab.
137
138       Control characters
139
140       "\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
147       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
151       To write platform-independent code, you must use "\N{NAME}" instead,
152       like "\N{ESCAPE}" or "\N{U+001B}", see charnames.
153
154       Mnemonic: control character.
155
156       Example
157
158        $str =~ /\cK/;  # Matches if $str contains a vertical tab (control-K).
159
160       Named or numbered characters and character sequences
161
162       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
166       To specify by name, the name of the character or character sequence
167       goes between the curly braces.
168
169       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
178       It is even possible to give your own names to characters and character
179       sequences by using the charnames module.  These custom names are
180       lexically scoped, and so a given code point may have different names in
181       different scopes.  The name used is what is in effect at the time the
182       "\N{}" is expanded.  For patterns in double-quotish context, that means
183       at the time the pattern is parsed.  But for patterns that are
184       delimitted by single quotes, the expansion is deferred until pattern
185       compilation time, which may very well have a different "charnames"
186       translator in effect.
187
188       (There is an expanded internal form that you may see in debug output:
189       "\N{U+code point.code point...}".  The "..." means any number of these
190       code points separated by dots.  This represents the sequence formed by
191       the characters.  This is an internal form only, subject to change, and
192       you should not try to use it yourself.)
193
194       Mnemonic: Named character.
195
196       Note that a character or character sequence expressed as a named or
197       numbered character is considered a character without special meaning by
198       the regex engine, and will match "as is".
199
200       Example
201
202        $str =~ /\N{THAI CHARACTER SO SO}/;  # Matches the Thai SO SO character
203
204        use charnames 'Cyrillic';            # Loads Cyrillic names.
205        $str =~ /\N{ZHE}\N{KA}/;             # Match "ZHE" followed by "KA".
206
207       Octal escapes
208
209       There are two forms of octal escapes.  Each is used to specify a
210       character by its code point specified in octal notation.
211
212       One form, available starting in Perl 5.14 looks like "\o{...}", where
213       the dots represent one or more octal digits.  It can be used for any
214       Unicode character.
215
216       It was introduced to avoid the potential problems with the other form,
217       available in all Perls.  That form consists of a backslash followed by
218       three octal digits.  One problem with this form is that it can look
219       exactly like an old-style backreference (see "Disambiguation rules
220       between old-style octal escapes and backreferences" below.)  You can
221       avoid this by making the first of the three digits always a zero, but
222       that makes \077 the largest code point specifiable.
223
224       In some contexts, a backslash followed by two or even one octal digits
225       may be interpreted as an octal escape, sometimes with a warning, and
226       because of some bugs, sometimes with surprising results.  Also, if you
227       are creating a regex out of smaller snippets concatenated together, and
228       you use fewer than three digits, the beginning of one snippet may be
229       interpreted as adding digits to the ending of the snippet before it.
230       See "Absolute referencing" for more discussion and examples of the
231       snippet problem.
232
233       Note that a character expressed as an octal escape is considered a
234       character without special meaning by the regex engine, and will match
235       "as is".
236
237       To summarize, the "\o{}" form is always safe to use, and the other form
238       is safe to use for code points through \077 when you use exactly three
239       digits to specify them.
240
241       Mnemonic: 0ctal or octal.
242
243       Examples (assuming an ASCII platform)
244
245        $str = "Perl";
246        $str =~ /\o{120}/;  # Match, "\120" is "P".
247        $str =~ /\120/;     # Same.
248        $str =~ /\o{120}+/; # Match, "\120" is "P",
249                            # it's repeated at least once.
250        $str =~ /\120+/;    # Same.
251        $str =~ /P\053/;    # No match, "\053" is "+" and taken literally.
252        /\o{23073}/         # Black foreground, white background smiling face.
253        /\o{4801234567}/    # Raises a warning, and yields chr(4).
254
255       Disambiguation rules between old-style octal escapes and backreferences
256
257       Octal escapes of the "\000" form outside of bracketed character classes
258       potentially clash with old-style backreferences (see "Absolute
259       referencing" below).  They both consist of a backslash followed by
260       numbers.  So Perl has to use heuristics to determine whether it is a
261       backreference or an octal escape.  Perl uses the following rules to
262       disambiguate:
263
264       1.  If the backslash is followed by a single digit, it's a
265           backreference.
266
267       2.  If the first digit following the backslash is a 0, it's an octal
268           escape.
269
270       3.  If the number following the backslash is N (in decimal), and Perl
271           already has seen N capture groups, Perl considers this a
272           backreference.  Otherwise, it considers it an octal escape. If N
273           has more than three digits, Perl takes only the first three for the
274           octal escape; the rest are matched as is.
275
276            my $pat  = "(" x 999;
277               $pat .= "a";
278               $pat .= ")" x 999;
279            /^($pat)\1000$/;   #  Matches 'aa'; there are 1000 capture groups.
280            /^$pat\1000$/;     #  Matches 'a@0'; there are 999 capture groups
281                               #  and \1000 is seen as \100 (a '@') and a '0'.
282
283       You can force a backreference interpretation always by using the
284       "\g{...}" form.  You can the force an octal interpretation always by
285       using the "\o{...}" form, or for numbers up through \077 (= 63
286       decimal), by using three digits, beginning with a "0".
287
288       Hexadecimal escapes
289
290       Like octal escapes, there are two forms of hexadecimal escapes, but
291       both start with the sequence "\x".  This is followed by either exactly
292       two hexadecimal digits forming a number, or a hexadecimal number of
293       arbitrary length surrounded by curly braces. The hexadecimal number is
294       the code point of the character you want to express.
295
296       Note that a character expressed as one of these escapes is considered a
297       character without special meaning by the regex engine, and will match
298       "as is".
299
300       Mnemonic: hexadecimal.
301
302       Examples (assuming an ASCII platform)
303
304        $str = "Perl";
305        $str =~ /\x50/;    # Match, "\x50" is "P".
306        $str =~ /\x50+/;   # Match, "\x50" is "P", it is repeated at least once
307        $str =~ /P\x2B/;   # No match, "\x2B" is "+" and taken literally.
308
309        /\x{2603}\x{2602}/ # Snowman with an umbrella.
310                           # The Unicode character 2603 is a snowman,
311                           # the Unicode character 2602 is an umbrella.
312        /\x{263B}/         # Black smiling face.
313        /\x{263b}/         # Same, the hex digits A - F are case insensitive.
314
315   Modifiers
316       A number of backslash sequences have to do with changing the character,
317       or characters following them. "\l" will lowercase the character
318       following it, while "\u" will uppercase (or, more accurately,
319       titlecase) the character following it. They provide functionality
320       similar to the functions "lcfirst" and "ucfirst".
321
322       To uppercase or lowercase several characters, one might want to use
323       "\L" or "\U", which will lowercase/uppercase all characters following
324       them, until either the end of the pattern or the next occurrence of
325       "\E", whichever comes first. They provide functionality similar to what
326       the functions "lc" and "uc" provide.
327
328       "\Q" is used to quote (disable) pattern metacharacters, up to the next
329       "\E" or the end of the pattern. "\Q" adds a backslash to any character
330       that could have special meaning to Perl.  In the ASCII range, it quotes
331       every character that isn't a letter, digit, or underscore.  See
332       "quotemeta" in perlfunc for details on what gets quoted for non-ASCII
333       code points.  Using this ensures that any character between "\Q" and
334       "\E" will be matched literally, not interpreted as a metacharacter by
335       the regex engine.
336
337       "\F" can be used to casefold all characters following, up to the next
338       "\E" or the end of the pattern. It provides the functionality similar
339       to the "fc" function.
340
341       Mnemonic: Lowercase, Uppercase, Fold-case, Quotemeta, End.
342
343       Examples
344
345        $sid     = "sid";
346        $greg    = "GrEg";
347        $miranda = "(Miranda)";
348        $str     =~ /\u$sid/;        # Matches 'Sid'
349        $str     =~ /\L$greg/;       # Matches 'greg'
350        $str     =~ /\Q$miranda\E/;  # Matches '(Miranda)', as if the pattern
351                                     #   had been written as /\(Miranda\)/
352
353   Character classes
354       Perl regular expressions have a large range of character classes. Some
355       of the character classes are written as a backslash sequence. We will
356       briefly discuss those here; full details of character classes can be
357       found in perlrecharclass.
358
359       "\w" is a character class that matches any single word character
360       (letters, digits, Unicode marks, and connector punctuation (like the
361       underscore)).  "\d" is a character class that matches any decimal
362       digit, while the character class "\s" matches any whitespace character.
363       New in perl 5.10.0 are the classes "\h" and "\v" which match horizontal
364       and vertical whitespace characters.
365
366       The exact set of characters matched by "\d", "\s", and "\w" varies
367       depending on various pragma and regular expression modifiers.  It is
368       possible to restrict the match to the ASCII range by using the "/a"
369       regular expression modifier.  See perlrecharclass.
370
371       The uppercase variants ("\W", "\D", "\S", "\H", and "\V") are character
372       classes that match, respectively, any character that isn't a word
373       character, digit, whitespace, horizontal whitespace, or vertical
374       whitespace.
375
376       Mnemonics: word, digit, space, horizontal, vertical.
377
378       Unicode classes
379
380       "\pP" (where "P" is a single letter) and "\p{Property}" are used to
381       match a character that matches the given Unicode property; properties
382       include things like "letter", or "thai character". Capitalizing the
383       sequence to "\PP" and "\P{Property}" make the sequence match a
384       character that doesn't match the given Unicode property. For more
385       details, see "Backslash sequences" in perlrecharclass and "Unicode
386       Character Properties" in perlunicode.
387
388       Mnemonic: property.
389
390   Referencing
391       If capturing parenthesis are used in a regular expression, we can refer
392       to the part of the source string that was matched, and match exactly
393       the same thing. There are three ways of referring to such
394       backreference: absolutely, relatively, and by name.
395
396       Absolute referencing
397
398       Either "\gN" (starting in Perl 5.10.0), or "\N" (old-style) where N is
399       a positive (unsigned) decimal number of any length is an absolute
400       reference to a capturing group.
401
402       N refers to the Nth set of parentheses, so "\gN" refers to whatever has
403       been matched by that set of parentheses.  Thus "\g1" refers to the
404       first capture group in the regex.
405
406       The "\gN" form can be equivalently written as "\g{N}" which avoids
407       ambiguity when building a regex by concatenating shorter strings.
408       Otherwise if you had a regex "qr/$a$b/", and $a contained "\g1", and $b
409       contained "37", you would get "/\g137/" which is probably not what you
410       intended.
411
412       In the "\N" form, N must not begin with a "0", and there must be at
413       least N capturing groups, or else N is considered an octal escape (but
414       something like "\18" is the same as "\0018"; that is, the octal escape
415       "\001" followed by a literal digit "8").
416
417       Mnemonic: group.
418
419       Examples
420
421        /(\w+) \g1/;    # Finds a duplicated word, (e.g. "cat cat").
422        /(\w+) \1/;     # Same thing; written old-style.
423        /(.)(.)\g2\g1/;  # Match a four letter palindrome (e.g. "ABBA").
424
425       Relative referencing
426
427       "\g-N" (starting in Perl 5.10.0) is used for relative addressing.  (It
428       can be written as "\g{-N}".)  It refers to the Nth group before the
429       "\g{-N}".
430
431       The big advantage of this form is that it makes it much easier to write
432       patterns with references that can be interpolated in larger patterns,
433       even if the larger pattern also contains capture groups.
434
435       Examples
436
437        /(A)        # Group 1
438         (          # Group 2
439           (B)      # Group 3
440           \g{-1}   # Refers to group 3 (B)
441           \g{-3}   # Refers to group 1 (A)
442         )
443        /x;         # Matches "ABBA".
444
445        my $qr = qr /(.)(.)\g{-2}\g{-1}/;  # Matches 'abab', 'cdcd', etc.
446        /$qr$qr/                           # Matches 'ababcdcd'.
447
448       Named referencing
449
450       "\g{name}" (starting in Perl 5.10.0) can be used to back refer to a
451       named capture group, dispensing completely with having to think about
452       capture buffer positions.
453
454       To be compatible with .Net regular expressions, "\g{name}" may also be
455       written as "\k{name}", "\k<name>" or "\k'name'".
456
457       To prevent any ambiguity, name must not start with a digit nor contain
458       a hyphen.
459
460       Examples
461
462        /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
463        /(?<word>\w+) \k{word}/ # Same.
464        /(?<word>\w+) \k<word>/ # Same.
465        /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
466                                # Match a four letter palindrome (e.g. "ABBA")
467
468   Assertions
469       Assertions are conditions that have to be true; they don't actually
470       match parts of the substring. There are six assertions that are written
471       as backslash sequences.
472
473       \A  "\A" only matches at the beginning of the string. If the "/m"
474           modifier isn't used, then "/\A/" is equivalent to "/^/". However,
475           if the "/m" modifier is used, then "/^/" matches internal newlines,
476           but the meaning of "/\A/" isn't changed by the "/m" modifier. "\A"
477           matches at the beginning of the string regardless whether the "/m"
478           modifier is used.
479
480       \z, \Z
481           "\z" and "\Z" match at the end of the string. If the "/m" modifier
482           isn't used, then "/\Z/" is equivalent to "/$/"; that is, it matches
483           at the end of the string, or one before the newline at the end of
484           the string. If the "/m" modifier is used, then "/$/" matches at
485           internal newlines, but the meaning of "/\Z/" isn't changed by the
486           "/m" modifier. "\Z" matches at the end of the string (or just
487           before a trailing newline) regardless whether the "/m" modifier is
488           used.
489
490           "\z" is just like "\Z", except that it does not match before a
491           trailing newline. "\z" matches at the end of the string only,
492           regardless of the modifiers used, and not just before a newline.
493           It is how to anchor the match to the true end of the string under
494           all conditions.
495
496       \G  "\G" is usually used only in combination with the "/g" modifier. If
497           the "/g" modifier is used and the match is done in scalar context,
498           Perl remembers where in the source string the last match ended, and
499           the next time, it will start the match from where it ended the
500           previous time.
501
502           "\G" matches the point where the previous match on that string
503           ended, or the beginning of that string if there was no previous
504           match.
505
506           Mnemonic: Global.
507
508       \b{}, \b, \B{}, \B
509           "\b{...}", available starting in v5.22, matches a boundary (between
510           two characters, or before the first character of the string, or
511           after the final character of the string) based on the Unicode rules
512           for the boundary type specified inside the braces.  The boundary
513           types are given a few paragraphs below.  "\B{...}" matches at any
514           place between characters where "\b{...}" of the same type doesn't
515           match.
516
517           "\b" when not immediately followed by a "{" matches at any place
518           between a word (something matched by "\w") and a non-word character
519           ("\W"); "\B" when not immediately followed by a "{" matches at any
520           place between characters where "\b" doesn't match.  To get better
521           word matching of natural language text, see "\b{wb}" below.
522
523           "\b" and "\B" assume there's a non-word character before the
524           beginning and after the end of the source string; so "\b" will
525           match at the beginning (or end) of the source string if the source
526           string begins (or ends) with a word character. Otherwise, "\B" will
527           match.
528
529           Do not use something like "\b=head\d\b" and expect it to match the
530           beginning of a line.  It can't, because for there to be a boundary
531           before the non-word "=", there must be a word character immediately
532           previous.  All plain "\b" and "\B" boundary determinations look for
533           word characters alone, not for non-word characters nor for string
534           ends.  It may help to understand how "\b" and "\B" work by equating
535           them as follows:
536
537               \b  really means    (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
538               \B  really means    (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
539
540           In contrast, "\b{...}" and "\B{...}" may or may not match at the
541           beginning and end of the line, depending on the boundary type.
542           These implement the Unicode default boundaries, specified in
543           <http://www.unicode.org/reports/tr14/> and
544           <http://www.unicode.org/reports/tr29/>.  The boundary types are:
545
546           "\b{gcb}" or "\b{g}"
547               This matches a Unicode "Grapheme Cluster Boundary".  (Actually
548               Perl always uses the improved "extended" grapheme cluster").
549               These are explained below under ""\X"".  In fact, "\X" is
550               another way to get the same functionality.  It is equivalent to
551               "/.+?\b{gcb}/".  Use whichever is most convenient for your
552               situation.
553
554           "\b{lb}"
555               This matches according to the default Unicode Line Breaking
556               Algorithm (<http://www.unicode.org/reports/tr14/>), as
557               customized in that document (Example 7 of revision 35
558               <http://www.unicode.org/reports/tr14/tr14-35.html#Example7>)
559               for better handling of numeric expressions.
560
561               This is suitable for many purposes, but the Unicode::LineBreak
562               module is available on CPAN that provides many more features,
563               including customization.
564
565           "\b{sb}"
566               This matches a Unicode "Sentence Boundary".  This is an aid to
567               parsing natural language sentences.  It gives good, but
568               imperfect results.  For example, it thinks that "Mr. Smith" is
569               two sentences.  More details are at
570               <http://www.unicode.org/reports/tr29/>.  Note also that it
571               thinks that anything matching "\R" (except form feed and
572               vertical tab) is a sentence boundary.  "\b{sb}" works with text
573               designed for word-processors which wrap lines automatically for
574               display, but hard-coded line boundaries are considered to be
575               essentially the ends of text blocks (paragraphs really), and
576               hence the ends of sentences.  "\b{sb}" doesn't do well with
577               text containing embedded newlines, like the source text of the
578               document you are reading.  Such text needs to be preprocessed
579               to get rid of the line separators before looking for sentence
580               boundaries.  Some people view this as a bug in the Unicode
581               standard, and this behavior is quite subject to change in
582               future Perl versions.
583
584           "\b{wb}"
585               This matches a Unicode "Word Boundary", but tailored to Perl
586               expectations.  This gives better (though not perfect) results
587               for natural language processing than plain "\b" (without
588               braces) does.  For example, it understands that apostrophes can
589               be in the middle of words and that parentheses aren't (see the
590               examples below).  More details are at
591               <http://www.unicode.org/reports/tr29/>.
592
593               The current Unicode definition of a Word Boundary matches
594               between every white space character.  Perl tailors this,
595               starting in version 5.24, to generally not break up spans of
596               white space, just as plain "\b" has always functioned.  This
597               allows "\b{wb}" to be a drop-in replacement for "\b", but with
598               generally better results for natural language processing.  (The
599               exception to this tailoring is when a span of white space is
600               immediately followed by something like U+0303, COMBINING TILDE.
601               If the final space character in the span is a horizontal white
602               space, it is broken out so that it attaches instead to the
603               combining character.  To be precise, if a span of white space
604               that ends in a horizontal space has the character immediately
605               following it have any of the Word Boundary property values
606               "Extend", "Format" or "ZWJ", the boundary between the final
607               horizontal space character and the rest of the span matches
608               "\b{wb}".  In all other cases the boundary between two white
609               space characters matches "\B{wb}".)
610
611           It is important to realize when you use these Unicode boundaries,
612           that you are taking a risk that a future version of Perl which
613           contains a later version of the Unicode Standard will not work
614           precisely the same way as it did when your code was written.  These
615           rules are not considered stable and have been somewhat more subject
616           to change than the rest of the Standard.  Unicode reserves the
617           right to change them at will, and Perl reserves the right to update
618           its implementation to Unicode's new rules.  In the past, some
619           changes have been because new characters have been added to the
620           Standard which have different characteristics than all previous
621           characters, so new rules are formulated for handling them.  These
622           should not cause any backward compatibility issues.  But some
623           changes have changed the treatment of existing characters because
624           the Unicode Technical Committee has decided that the change is
625           warranted for whatever reason.  This could be to fix a bug, or
626           because they think better results are obtained with the new rule.
627
628           It is also important to realize that these are default boundary
629           definitions, and that implementations may wish to tailor the
630           results for particular purposes and locales.  For example, some
631           languages, such as Japanese and Thai, require dictionary lookup to
632           accurately determine word boundaries.
633
634           Mnemonic: boundary.
635
636       Examples
637
638         "cat"   =~ /\Acat/;     # Match.
639         "cat"   =~ /cat\Z/;     # Match.
640         "cat\n" =~ /cat\Z/;     # Match.
641         "cat\n" =~ /cat\z/;     # No match.
642
643         "cat"   =~ /\bcat\b/;   # Matches.
644         "cats"  =~ /\bcat\b/;   # No match.
645         "cat"   =~ /\bcat\B/;   # No match.
646         "cats"  =~ /\bcat\B/;   # Match.
647
648         while ("cat dog" =~ /(\w+)/g) {
649             print $1;           # Prints 'catdog'
650         }
651         while ("cat dog" =~ /\G(\w+)/g) {
652             print $1;           # Prints 'cat'
653         }
654
655         my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
656         print join("|", $s =~ m/ ( .+? \b     ) /xg), "\n";
657         print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
658        prints
659         He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
660         He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"
661
662   Misc
663       Here we document the backslash sequences that don't fall in one of the
664       categories above. These are:
665
666       \K  This appeared in perl 5.10.0. Anything matched left of "\K" is not
667           included in $&, and will not be replaced if the pattern is used in
668           a substitution. This lets you write "s/PAT1 \K PAT2/REPL/x" instead
669           of "s/(PAT1) PAT2/${1}REPL/x" or "s/(?<=PAT1) PAT2/REPL/x".
670
671           Mnemonic: Keep.
672
673       \N  This feature, available starting in v5.12,  matches any character
674           that is not a newline.  It is a short-hand for writing "[^\n]", and
675           is identical to the "." metasymbol, except under the "/s" flag,
676           which changes the meaning of ".", but not "\N".
677
678           Note that "\N{...}" can mean a named or numbered character .
679
680           Mnemonic: Complement of \n.
681
682       \R  "\R" matches a generic newline; that is, anything considered a
683           linebreak sequence by Unicode. This includes all characters matched
684           by "\v" (vertical whitespace), and the multi character sequence
685           "\x0D\x0A" (carriage return followed by a line feed, sometimes
686           called the network newline; it's the end of line sequence used in
687           Microsoft text files opened in binary mode). "\R" is equivalent to
688           "(?>\x0D\x0A|\v)".  (The reason it doesn't backtrack is that the
689           sequence is considered inseparable.  That means that
690
691            "\x0D\x0A" =~ /^\R\x0A$/   # No match
692
693           fails, because the "\R" matches the entire string, and won't
694           backtrack to match just the "\x0D".)  Since "\R" can match a
695           sequence of more than one character, it cannot be put inside a
696           bracketed character class; "/[\R]/" is an error; use "\v" instead.
697           "\R" was introduced in perl 5.10.0.
698
699           Note that this does not respect any locale that might be in effect;
700           it matches according to the platform's native character set.
701
702           Mnemonic: none really. "\R" was picked because PCRE already uses
703           "\R", and more importantly because Unicode recommends such a
704           regular expression metacharacter, and suggests "\R" as its
705           notation.
706
707       \X  This matches a Unicode extended grapheme cluster.
708
709           "\X" matches quite well what normal (non-Unicode-programmer) usage
710           would consider a single character.  As an example, consider a G
711           with some sort of diacritic mark, such as an arrow.  There is no
712           such single character in Unicode, but one can be composed by using
713           a G followed by a Unicode "COMBINING UPWARDS ARROW BELOW", and
714           would be displayed by Unicode-aware software as if it were a single
715           character.
716
717           The match is greedy and non-backtracking, so that the cluster is
718           never broken up into smaller components.
719
720           See also "\b{gcb}".
721
722           Mnemonic: eXtended Unicode character.
723
724       Examples
725
726        $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
727        $str =~ s/(.)\K\g1//g;    # Delete duplicated characters.
728
729        "\n"   =~ /^\R$/;         # Match, \n   is a generic newline.
730        "\r"   =~ /^\R$/;         # Match, \r   is a generic newline.
731        "\r\n" =~ /^\R$/;         # Match, \r\n is a generic newline.
732
733        "P\x{307}" =~ /^\X$/     # \X matches a P with a dot above.
734
735
736
737perl v5.30.2                      2020-03-27                PERLREBACKSLASH(1)
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