3 perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
7 The top level documentation about Perl regular expressions
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.
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.
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.
26 The rules determining what it is are quite simple: if the character
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 any special meaning of the character following it.
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
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 issues a warning if you use such a
38 It is however guaranteed that backslash or escape sequences never have a
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
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
51 There is one exception. If you use an alphanumeric character as the
52 delimiter of your pattern (which you probably shouldn't do for readability
53 reasons), you 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
60 =head2 All the sequences and escapes
62 Those not usable within a bracketed character class (like C<[\da-z]>) are marked
65 \000 Octal escape sequence. See also \o{}.
66 \1 Absolute backreference. Not in [].
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 [].
72 \C Single octet, even under UTF-8. Not in [].
73 \d Character class for digits.
74 \D Character class for non-digits.
76 \E Turn off \Q, \L and \U processing. Not in [].
78 \g{}, \g1 Named, absolute or relative backreference. Not in []
79 \G Pos assertion. Not in [].
80 \h Character class for horizontal whitespace.
81 \H Character class for non horizontal whitespace.
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 [].
86 \n (Logical) newline character.
87 \N Any character but newline. Experimental. Not in [].
88 \N{} Named or numbered (Unicode) character or sequence.
89 \o{} Octal escape sequence.
90 \p{}, \pP Character with the given Unicode property.
91 \P{}, \PP Character without the given Unicode property.
92 \Q Quotemeta till \E. Not in [].
94 \R Generic new line. Not in [].
95 \s Character class for whitespace.
96 \S Character class for non whitespace.
98 \u Titlecase next character. Not in [].
99 \U Uppercase till \E. Not in [].
100 \v Character class for vertical whitespace.
101 \V Character class for non vertical whitespace.
102 \w Character class for word characters.
103 \W Character class for non-word characters.
104 \x{}, \x00 Hexadecimal escape sequence.
105 \X Unicode "extended grapheme cluster". Not in [].
106 \z End of string. Not in [].
107 \Z End of string. Not in [].
109 =head2 Character Escapes
111 =head3 Fixed characters
113 A handful of characters have a dedicated I<character escape>. The following
114 table shows them, along with their ASCII code points (in decimal and hex),
115 their ASCII name, the control escape on ASCII platforms and a short
116 description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
118 Seq. Code Point ASCII Cntrl Description.
120 \a 7 07 BEL \cG alarm or bell
121 \b 8 08 BS \cH backspace [1]
122 \e 27 1B ESC \c[ escape character
123 \f 12 0C FF \cL form feed
124 \n 10 0A LF \cJ line feed [2]
125 \r 13 0D CR \cM carriage return
132 C<\b> is the backspace character only inside a character class. Outside a
133 character class, C<\b> is a word/non-word boundary.
137 C<\n> matches a logical newline. Perl converts between C<\n> and your
138 OS's native newline character when reading from or writing to text files.
144 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
146 =head3 Control characters
148 C<\c> is used to denote a control character; the character following C<\c>
149 determines the value of the construct. For example the value of C<\cA> is
150 C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
151 The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
152 list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
153 L<perlebcdic/OPERATOR DIFFERENCES>.
155 Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
156 string) is not valid. The backslash must be followed by another character.
157 That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
159 To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
160 C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
162 Mnemonic: I<c>ontrol character.
166 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
168 =head3 Named or numbered characters and character sequences
170 Unicode characters have a Unicode name and numeric code point (ordinal)
172 C<\N{}> construct to specify a character by either of these values.
173 Certain sequences of characters also have names.
175 To specify by name, the name of the character or character sequence goes
176 between the curly braces. In this case, you have to C<use charnames> to
177 load the Unicode names of the characters; otherwise Perl will complain.
179 To specify a character by Unicode code point, use the form C<\N{U+I<code
180 point>}>, where I<code point> is a number in hexadecimal that gives the
181 code point that Unicode has assigned to the desired character. It is
182 customary but not required to use leading zeros to pad the number to 4
183 digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
184 rarely see it written without the two leading zeros. C<\N{U+0041}> means
185 "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
187 It is even possible to give your own names to characters and character
188 sequences. For details, see L<charnames>.
190 (There is an expanded internal form that you may see in debug output:
191 C<\N{U+I<code point>.I<code point>...}>.
192 The C<...> means any number of these I<code point>s separated by dots.
193 This represents the sequence formed by the characters. This is an internal
194 form only, subject to change, and you should not try to use it yourself.)
196 Mnemonic: I<N>amed character.
198 Note that a character or character sequence expressed as a named
199 or numbered character is considered a character without special
200 meaning by the regex engine, and will match "as is".
204 use charnames ':full'; # Loads the Unicode names.
205 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
207 use charnames 'Cyrillic'; # Loads Cyrillic names.
208 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
212 There are two forms of octal escapes. Each is used to specify a character by
213 its code point specified in octal notation.
215 One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
216 represent one or more octal digits. It can be used for any Unicode character.
218 It was introduced to avoid the potential problems with the other form,
219 available in all Perls. That form consists of a backslash followed by three
220 octal digits. One problem with this form is that it can look exactly like an
221 old-style backreference (see
222 L</Disambiguation rules between old-style octal escapes and backreferences>
223 below.) You can avoid this by making the first of the three digits always a
224 zero, but that makes \077 the largest code point specifiable.
226 In some contexts, a backslash followed by two or even one octal digits may be
227 interpreted as an octal escape, sometimes with a warning, and because of some
228 bugs, sometimes with surprising results. Also, if you are creating a regex
229 out of smaller snippets concatenated together, and you use fewer than three
230 digits, the beginning of one snippet may be interpreted as adding digits to the
231 ending of the snippet before it. See L</Absolute referencing> for more
232 discussion and examples of the snippet problem.
234 Note that a character expressed as an octal escape is considered
235 a character without special meaning by the regex engine, and will match
238 To summarize, the C<\o{}> form is always safe to use, and the other form is
239 safe to use for code points through \077 when you use exactly three digits to
242 Mnemonic: I<0>ctal or I<o>ctal.
244 =head4 Examples (assuming an ASCII platform)
247 $str =~ /\o{120}/; # Match, "\120" is "P".
248 $str =~ /\120/; # Same.
249 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
250 $str =~ /\120+/; # Same.
251 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
252 /\o{23073}/ # Black foreground, white background smiling face.
253 /\o{4801234567}/ # Raises a warning, and yields chr(4)
255 =head4 Disambiguation rules between old-style octal escapes and backreferences
257 Octal escapes of the C<\000> form outside of bracketed character classes
258 potentially clash with old-style backreferences. (see L</Absolute referencing>
259 below). They both consist of a backslash followed by numbers. So Perl has to
260 use heuristics to determine whether it is a backreference or an octal escape.
261 Perl uses the following rules to disambiguate:
267 If the backslash is followed by a single digit, it's a backreference.
271 If the first digit following the backslash is a 0, it's an octal escape.
275 If the number following the backslash is N (in decimal), and Perl already
276 has seen N capture groups, Perl considers this a backreference. Otherwise,
277 it considers it an octal escape. If N has more than three digits, Perl
278 takes only the first three for the octal escape; the rest are matched as is.
283 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
284 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
285 # and \1000 is seen as \100 (a '@') and a '0'
289 You can force a backreference interpretation always by using the C<\g{...}>
290 form. You can the force an octal interpretation always by using the C<\o{...}>
291 form, or for numbers up through \077 (= 63 decimal), by using three digits,
292 beginning with a "0".
294 =head3 Hexadecimal escapes
296 Like octal escapes, there are two forms of hexadecimal escapes, but both start
297 with the same thing, C<\x>. This is followed by either exactly two hexadecimal
298 digits forming a number, or a hexadecimal number of arbitrary length surrounded
299 by curly braces. The hexadecimal number is the code point of the character you
302 Note that a character expressed as one of these escapes is considered a
303 character without special meaning by the regex engine, and will match
306 Mnemonic: heI<x>adecimal.
308 =head4 Examples (assuming an ASCII platform)
311 $str =~ /\x50/; # Match, "\x50" is "P".
312 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
313 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
315 /\x{2603}\x{2602}/ # Snowman with an umbrella.
316 # The Unicode character 2603 is a snowman,
317 # the Unicode character 2602 is an umbrella.
318 /\x{263B}/ # Black smiling face.
319 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
323 A number of backslash sequences have to do with changing the character,
324 or characters following them. C<\l> will lowercase the character following
325 it, while C<\u> will uppercase (or, more accurately, titlecase) the
326 character following it. They provide functionality similar to the
327 functions C<lcfirst> and C<ucfirst>.
329 To uppercase or lowercase several characters, one might want to use
330 C<\L> or C<\U>, which will lowercase/uppercase all characters following
331 them, until either the end of the pattern or the next occurrence of
332 C<\E>, whichever comes first. They provide functionality similar to what
333 the functions C<lc> and C<uc> provide.
335 C<\Q> is used to escape all characters following, up to the next C<\E>
336 or the end of the pattern. C<\Q> adds a backslash to any character that
337 isn't a letter, digit, or underscore. This ensures that any character
338 between C<\Q> and C<\E> shall be matched literally, not interpreted
339 as a metacharacter by the regex engine.
341 Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
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 /\(Miranda\)/
353 =head2 Character classes
355 Perl regular expressions have a large range of character classes. Some of
356 the character classes are written as a backslash sequence. We will briefly
357 discuss those here; full details of character classes can be found in
360 C<\w> is a character class that matches any single I<word> character
361 (letters, digits, Unicode marks, and connector punctuation (like the
362 underscore)). C<\d> is a character class that matches any decimal
363 digit, while the character class C<\s> matches any whitespace character.
364 New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
365 and vertical whitespace characters.
367 The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
368 depending on various pragma and regular expression modifiers. It is
369 possible to restrict the match to the ASCII range by using the C</a>
370 regular expression modifier. See L<perlrecharclass>.
372 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
373 character classes that match, respectively, any character that isn't a
374 word character, digit, whitespace, horizontal whitespace, or vertical
377 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
379 =head3 Unicode classes
381 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
382 match a character that matches the given Unicode property; properties
383 include things like "letter", or "thai character". Capitalizing the
384 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
385 that doesn't match the given Unicode property. For more details, see
386 L<perlrecharclass/Backslash sequences> and
387 L<perlunicode/Unicode Character Properties>.
389 Mnemonic: I<p>roperty.
393 If capturing parenthesis are used in a regular expression, we can refer
394 to the part of the source string that was matched, and match exactly the
395 same thing. There are three ways of referring to such I<backreference>:
396 absolutely, relatively, and by name.
398 =for later add link to perlrecapture
400 =head3 Absolute referencing
402 Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
403 is a positive (unsigned) decimal number of any length is an absolute reference
404 to a capturing group.
406 I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
407 been matched by that set of parentheses. Thus C<\g1> refers to the first
408 capture group in the regex.
410 The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
411 which avoids ambiguity when building a regex by concatenating shorter
412 strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
413 C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
414 probably not what you intended.
416 In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
417 least I<N> capturing groups, or else I<N> is considered an octal escape
418 (but something like C<\18> is the same as C<\0018>; that is, the octal escape
419 C<"\001"> followed by a literal digit C<"8">).
425 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
426 /(\w+) \1/; # Same thing; written old-style
427 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
430 =head3 Relative referencing
432 C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
433 be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
436 The big advantage of this form is that it makes it much easier to write
437 patterns with references that can be interpolated in larger patterns,
438 even if the larger pattern also contains capture groups.
445 \g{-1} # Refers to group 3 (B)
446 \g{-3} # Refers to group 1 (A)
448 /x; # Matches "ABBA".
450 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
451 /$qr$qr/ # Matches 'ababcdcd'.
453 =head3 Named referencing
455 C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
456 named capture group, dispensing completely with having to think about capture
459 To be compatible with .Net regular expressions, C<\g{name}> may also be
460 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
462 To prevent any ambiguity, I<name> must not start with a digit nor contain a
467 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
468 /(?<word>\w+) \k{word}/ # Same.
469 /(?<word>\w+) \k<word>/ # Same.
470 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
471 # Match a four letter palindrome (e.g. "ABBA")
475 Assertions are conditions that have to be true; they don't actually
476 match parts of the substring. There are six assertions that are written as
483 C<\A> only matches at the beginning of the string. If the C</m> modifier
484 isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
485 modifier is used, then C</^/> matches internal newlines, but the meaning
486 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
487 of the string regardless whether the C</m> modifier is used.
491 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
492 used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
493 end of the string, or one before the newline at the end of the string. If the
494 C</m> modifier is used, then C</$/> matches at internal newlines, but the
495 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
496 the end of the string (or just before a trailing newline) regardless whether
497 the C</m> modifier is used.
499 C<\z> is just like C<\Z>, except that it does not match before a trailing
500 newline. C<\z> matches at the end of the string only, regardless of the
501 modifiers used, and not just before a newline. It is how to anchor the
502 match to the true end of the string under all conditions.
506 C<\G> is usually used only in combination with the C</g> modifier. If the
507 C</g> modifier is used and the match is done in scalar context, Perl
508 remembers where in the source string the last match ended, and the next time,
509 it will start the match from where it ended the previous time.
511 C<\G> matches the point where the previous match on that string ended,
512 or the beginning of that string if there was no previous match.
514 =for later add link to perlremodifiers
520 C<\b> matches at any place between a word and a non-word character; C<\B>
521 matches at any place between characters where C<\b> doesn't match. C<\b>
522 and C<\B> assume there's a non-word character before the beginning and after
523 the end of the source string; so C<\b> will match at the beginning (or end)
524 of the source string if the source string begins (or ends) with a word
525 character. Otherwise, C<\B> will match.
527 Do not use something like C<\b=head\d\b> and expect it to match the
528 beginning of a line. It can't, because for there to be a boundary before
529 the non-word "=", there must be a word character immediately previous.
530 All boundary determinations look for word characters alone, not for
531 non-words characters nor for string ends. It may help to understand how
532 <\b> and <\B> work by equating them as follows:
534 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
535 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
537 Mnemonic: I<b>oundary.
543 "cat" =~ /\Acat/; # Match.
544 "cat" =~ /cat\Z/; # Match.
545 "cat\n" =~ /cat\Z/; # Match.
546 "cat\n" =~ /cat\z/; # No match.
548 "cat" =~ /\bcat\b/; # Matches.
549 "cats" =~ /\bcat\b/; # No match.
550 "cat" =~ /\bcat\B/; # No match.
551 "cats" =~ /\bcat\B/; # Match.
553 while ("cat dog" =~ /(\w+)/g) {
554 print $1; # Prints 'catdog'
556 while ("cat dog" =~ /\G(\w+)/g) {
557 print $1; # Prints 'cat'
562 Here we document the backslash sequences that don't fall in one of the
563 categories above. These are:
569 C<\C> always matches a single octet, even if the source string is encoded
570 in UTF-8 format, and the character to be matched is a multi-octet character.
571 C<\C> was introduced in perl 5.6. This is very dangerous, because it violates
572 the logical character abstraction and can cause UTF-8 sequences to become malformed.
578 This appeared in perl 5.10.0. Anything matched left of C<\K> is
579 not included in C<$&>, and will not be replaced if the pattern is
580 used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
581 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
587 This is an experimental feature new to perl 5.12.0. It matches any character
588 that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
589 identical to the C<.> metasymbol, except under the C</s> flag, which changes
590 the meaning of C<.>, but not C<\N>.
592 Note that C<\N{...}> can mean a
593 L<named or numbered character
594 |/Named or numbered characters and character sequences>.
596 Mnemonic: Complement of I<\n>.
601 C<\R> matches a I<generic newline>; that is, anything considered a
602 linebreak sequence by Unicode. This includes all characters matched by
603 C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
604 (carriage return followed by a line feed, sometimes called the network
605 newline; it's the end of line sequence used in Microsoft text files opened
606 in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. Since
607 C<\R> can match a sequence of more than one character, it cannot be put
608 inside a bracketed character class; C</[\R]/> is an error; use C<\v>
609 instead. C<\R> was introduced in perl 5.10.0.
611 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
612 and more importantly because Unicode recommends such a regular expression
613 metacharacter, and suggests C<\R> as its notation.
618 This matches a Unicode I<extended grapheme cluster>.
620 C<\X> matches quite well what normal (non-Unicode-programmer) usage
621 would consider a single character. As an example, consider a G with some sort
622 of diacritic mark, such as an arrow. There is no such single character in
623 Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
624 UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
625 were a single character.
627 Mnemonic: eI<X>tended Unicode character.
633 "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
635 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
636 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
638 "\n" =~ /^\R$/; # Match, \n is a generic newline.
639 "\r" =~ /^\R$/; # Match, \r is a generic newline.
640 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
642 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.