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 \F Foldcase till \E. Not in [].
79 \g{}, \g1 Named, absolute or relative backreference. Not in []
80 \G Pos assertion. Not in [].
81 \h Character class for horizontal whitespace.
82 \H Character class for non horizontal whitespace.
83 \k{}, \k<>, \k'' Named backreference. Not in [].
84 \K Keep the stuff left of \K. Not in [].
85 \l Lowercase next character. Not in [].
86 \L Lowercase till \E. Not in [].
87 \n (Logical) newline character.
88 \N Any character but newline. Experimental. Not in [].
89 \N{} Named or numbered (Unicode) character or sequence.
90 \o{} Octal escape sequence.
91 \p{}, \pP Character with the given Unicode property.
92 \P{}, \PP Character without the given Unicode property.
93 \Q Quote (disable) pattern metacharacters till \E. Not
96 \R Generic new line. Not in [].
97 \s Character class for whitespace.
98 \S Character class for non whitespace.
100 \u Titlecase next character. Not in [].
101 \U Uppercase till \E. Not in [].
102 \v Character class for vertical whitespace.
103 \V Character class for non vertical whitespace.
104 \w Character class for word characters.
105 \W Character class for non-word characters.
106 \x{}, \x00 Hexadecimal escape sequence.
107 \X Unicode "extended grapheme cluster". Not in [].
108 \z End of string. Not in [].
109 \Z End of string. Not in [].
111 =head2 Character Escapes
113 =head3 Fixed characters
115 A handful of characters have a dedicated I<character escape>. The following
116 table shows them, along with their ASCII code points (in decimal and hex),
117 their ASCII name, the control escape on ASCII platforms and a short
118 description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
120 Seq. Code Point ASCII Cntrl Description.
122 \a 7 07 BEL \cG alarm or bell
123 \b 8 08 BS \cH backspace [1]
124 \e 27 1B ESC \c[ escape character
125 \f 12 0C FF \cL form feed
126 \n 10 0A LF \cJ line feed [2]
127 \r 13 0D CR \cM carriage return
134 C<\b> is the backspace character only inside a character class. Outside a
135 character class, C<\b> is a word/non-word boundary.
139 C<\n> matches a logical newline. Perl converts between C<\n> and your
140 OS's native newline character when reading from or writing to text files.
146 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
148 =head3 Control characters
150 C<\c> is used to denote a control character; the character following C<\c>
151 determines the value of the construct. For example the value of C<\cA> is
152 C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
153 The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
154 list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
155 L<perlebcdic/OPERATOR DIFFERENCES>.
157 Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
158 string) is not valid. The backslash must be followed by another character.
159 That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
161 To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
162 C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
164 Mnemonic: I<c>ontrol character.
168 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
170 =head3 Named or numbered characters and character sequences
172 Unicode characters have a Unicode name and numeric code point (ordinal)
174 C<\N{}> construct to specify a character by either of these values.
175 Certain sequences of characters also have names.
177 To specify by name, the name of the character or character sequence goes
178 between the curly braces.
180 To specify a character by Unicode code point, use the form C<\N{U+I<code
181 point>}>, where I<code point> is a number in hexadecimal that gives the
182 code point that Unicode has assigned to the desired character. It is
183 customary but not required to use leading zeros to pad the number to 4
184 digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
185 rarely see it written without the two leading zeros. C<\N{U+0041}> means
186 "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
188 It is even possible to give your own names to characters and character
189 sequences. For details, see L<charnames>.
191 (There is an expanded internal form that you may see in debug output:
192 C<\N{U+I<code point>.I<code point>...}>.
193 The C<...> means any number of these I<code point>s separated by dots.
194 This represents the sequence formed by the characters. This is an internal
195 form only, subject to change, and you should not try to use it yourself.)
197 Mnemonic: I<N>amed character.
199 Note that a character or character sequence expressed as a named
200 or numbered character is considered a character without special
201 meaning by the regex engine, and will match "as is".
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 quote (disable) pattern metacharacters, up to the next
336 C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
337 that could have special meaning to Perl. In the ASCII range, it quotes
338 every character that isn't a letter, digit, or underscore. See
339 L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
340 code points. Using this ensures that any character between C<\Q> and
341 C<\E> will be matched literally, not interpreted as a metacharacter by
344 C<\F> can be used to casefold all characters following, up to the next C<\E>
345 or the end of the pattern. It provides the functionality similar to
348 Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
354 $miranda = "(Miranda)";
355 $str =~ /\u$sid/; # Matches 'Sid'
356 $str =~ /\L$greg/; # Matches 'greg'
357 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
358 # had been written as /\(Miranda\)/
360 =head2 Character classes
362 Perl regular expressions have a large range of character classes. Some of
363 the character classes are written as a backslash sequence. We will briefly
364 discuss those here; full details of character classes can be found in
367 C<\w> is a character class that matches any single I<word> character
368 (letters, digits, Unicode marks, and connector punctuation (like the
369 underscore)). C<\d> is a character class that matches any decimal
370 digit, while the character class C<\s> matches any whitespace character.
371 New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
372 and vertical whitespace characters.
374 The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
375 depending on various pragma and regular expression modifiers. It is
376 possible to restrict the match to the ASCII range by using the C</a>
377 regular expression modifier. See L<perlrecharclass>.
379 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
380 character classes that match, respectively, any character that isn't a
381 word character, digit, whitespace, horizontal whitespace, or vertical
384 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
386 =head3 Unicode classes
388 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
389 match a character that matches the given Unicode property; properties
390 include things like "letter", or "thai character". Capitalizing the
391 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
392 that doesn't match the given Unicode property. For more details, see
393 L<perlrecharclass/Backslash sequences> and
394 L<perlunicode/Unicode Character Properties>.
396 Mnemonic: I<p>roperty.
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 the
402 same thing. There are three ways of referring to such I<backreference>:
403 absolutely, relatively, and by name.
405 =for later add link to perlrecapture
407 =head3 Absolute referencing
409 Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
410 is a positive (unsigned) decimal number of any length is an absolute reference
411 to a capturing group.
413 I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
414 been matched by that set of parentheses. Thus C<\g1> refers to the first
415 capture group in the regex.
417 The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
418 which avoids ambiguity when building a regex by concatenating shorter
419 strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
420 C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
421 probably not what you intended.
423 In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
424 least I<N> capturing groups, or else I<N> is considered an octal escape
425 (but something like C<\18> is the same as C<\0018>; that is, the octal escape
426 C<"\001"> followed by a literal digit C<"8">).
432 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
433 /(\w+) \1/; # Same thing; written old-style
434 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
437 =head3 Relative referencing
439 C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
440 be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
443 The big advantage of this form is that it makes it much easier to write
444 patterns with references that can be interpolated in larger patterns,
445 even if the larger pattern also contains capture groups.
452 \g{-1} # Refers to group 3 (B)
453 \g{-3} # Refers to group 1 (A)
455 /x; # Matches "ABBA".
457 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
458 /$qr$qr/ # Matches 'ababcdcd'.
460 =head3 Named referencing
462 C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
463 named capture group, dispensing completely with having to think about capture
466 To be compatible with .Net regular expressions, C<\g{name}> may also be
467 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
469 To prevent any ambiguity, I<name> must not start with a digit nor contain a
474 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
475 /(?<word>\w+) \k{word}/ # Same.
476 /(?<word>\w+) \k<word>/ # Same.
477 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
478 # Match a four letter palindrome (e.g. "ABBA")
482 Assertions are conditions that have to be true; they don't actually
483 match parts of the substring. There are six assertions that are written as
490 C<\A> only matches at the beginning of the string. If the C</m> modifier
491 isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
492 modifier is used, then C</^/> matches internal newlines, but the meaning
493 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
494 of the string regardless whether the C</m> modifier is used.
498 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
499 used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
500 end of the string, or one before the newline at the end of the string. If the
501 C</m> modifier is used, then C</$/> matches at internal newlines, but the
502 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
503 the end of the string (or just before a trailing newline) regardless whether
504 the C</m> modifier is used.
506 C<\z> is just like C<\Z>, except that it does not match before a trailing
507 newline. C<\z> matches at the end of the string only, regardless of the
508 modifiers used, and not just before a newline. It is how to anchor the
509 match to the true end of the string under all conditions.
513 C<\G> is usually used only in combination with the C</g> modifier. If the
514 C</g> modifier is used and the match is done in scalar context, Perl
515 remembers where in the source string the last match ended, and the next time,
516 it will start the match from where it ended the previous time.
518 C<\G> matches the point where the previous match on that string ended,
519 or the beginning of that string if there was no previous match.
521 =for later add link to perlremodifiers
527 C<\b> matches at any place between a word and a non-word character; C<\B>
528 matches at any place between characters where C<\b> doesn't match. C<\b>
529 and C<\B> assume there's a non-word character before the beginning and after
530 the end of the source string; so C<\b> will match at the beginning (or end)
531 of the source string if the source string begins (or ends) with a word
532 character. Otherwise, C<\B> will match.
534 Do not use something like C<\b=head\d\b> and expect it to match the
535 beginning of a line. It can't, because for there to be a boundary before
536 the non-word "=", there must be a word character immediately previous.
537 All boundary determinations look for word characters alone, not for
538 non-words characters nor for string ends. It may help to understand how
539 <\b> and <\B> work by equating them as follows:
541 \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
542 \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
544 Mnemonic: I<b>oundary.
550 "cat" =~ /\Acat/; # Match.
551 "cat" =~ /cat\Z/; # Match.
552 "cat\n" =~ /cat\Z/; # Match.
553 "cat\n" =~ /cat\z/; # No match.
555 "cat" =~ /\bcat\b/; # Matches.
556 "cats" =~ /\bcat\b/; # No match.
557 "cat" =~ /\bcat\B/; # No match.
558 "cats" =~ /\bcat\B/; # Match.
560 while ("cat dog" =~ /(\w+)/g) {
561 print $1; # Prints 'catdog'
563 while ("cat dog" =~ /\G(\w+)/g) {
564 print $1; # Prints 'cat'
569 Here we document the backslash sequences that don't fall in one of the
570 categories above. These are:
576 C<\C> always matches a single octet, even if the source string is encoded
577 in UTF-8 format, and the character to be matched is a multi-octet character.
578 This is very dangerous, because it violates
579 the logical character abstraction and can cause UTF-8 sequences to become malformed.
585 This appeared in perl 5.10.0. Anything matched left of C<\K> is
586 not included in C<$&>, and will not be replaced if the pattern is
587 used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
588 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
594 This feature, available starting in v5.12, matches any character
595 that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
596 identical to the C<.> metasymbol, except under the C</s> flag, which changes
597 the meaning of C<.>, but not C<\N>.
599 Note that C<\N{...}> can mean a
600 L<named or numbered character
601 |/Named or numbered characters and character sequences>.
603 Mnemonic: Complement of I<\n>.
608 C<\R> matches a I<generic newline>; that is, anything considered a
609 linebreak sequence by Unicode. This includes all characters matched by
610 C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
611 (carriage return followed by a line feed, sometimes called the network
612 newline; it's the end of line sequence used in Microsoft text files opened
613 in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
614 reason it doesn't backtrack is that the sequence is considered
615 inseparable. That means that
617 "\x0D\x0A" =~ /^\R\x0A$/ # No match
619 fails, because the C<\R> matches the entire string, and won't backtrack
620 to match just the C<"\x0D">.) Since
621 C<\R> can match a sequence of more than one character, it cannot be put
622 inside a bracketed character class; C</[\R]/> is an error; use C<\v>
623 instead. C<\R> was introduced in perl 5.10.0.
625 Note that this does not respect any locale that might be in effect; it
626 matches according to the platform's native character set.
628 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
629 and more importantly because Unicode recommends such a regular expression
630 metacharacter, and suggests C<\R> as its notation.
635 This matches a Unicode I<extended grapheme cluster>.
637 C<\X> matches quite well what normal (non-Unicode-programmer) usage
638 would consider a single character. As an example, consider a G with some sort
639 of diacritic mark, such as an arrow. There is no such single character in
640 Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
641 UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
642 were a single character.
644 Mnemonic: eI<X>tended Unicode character.
650 "\x{256}" =~ /^\C\C$/; # Match as chr (0x256) takes 2 octets in UTF-8.
652 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
653 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
655 "\n" =~ /^\R$/; # Match, \n is a generic newline.
656 "\r" =~ /^\R$/; # Match, \r is a generic newline.
657 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
659 "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.