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PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1)
NAME
perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
DESCRIPTION
The top level documentation about Perl regular expressions is found in perlre.
This document describes all backslash and escape sequences. After explaining the role of the back-slash, backslash,
slash, it lists all the sequences that have a special meaning in Perl regular expressions (in alpha-betical alphabetical
betical order), then describes each of them.
Most sequences are described in detail in different documents; the primary purpose of this document
is to have a quick reference guide describing all backslash and escape sequences.
The backslash
In a regular expression, the backslash can perform one of two tasks: it either takes away the special
meaning of the character following it (for instance, "\|" matches a vertical bar, it's not an alter-nation), alternation),
nation), or it is the start of a backslash or escape sequence.
The rules determining what it is are quite simple: if the character following the backslash is a
punctuation (non-word) character (that is, anything that is not a letter, digit or underscore), then
the backslash just takes away the special meaning (if any) of the character following it.
If the character following the backslash is a letter or a digit, then the sequence may be special; if
so, it's listed below. A few letters have not been used yet, and escaping them with a backslash is
safe for now, but a future version of Perl may assign a special meaning to it. However, if you have
warnings turned on, Perl will issue a warning if you use such a sequence. [1].
It is however guaranteed that backslash or escape sequences never have a punctuation character fol-lowing following
lowing the backslash, not now, and not in a future version of Perl 5. So it is safe to put a back-slash backslash
slash in front of a non-word character.
Note that the backslash itself is special; if you want to match a backslash, you have to escape the
backslash with a backslash: "/\\/" matches a single backslash.
[1] There is one exception. If you use an alphanumerical character as the delimiter of your pattern
(which you probably shouldn't do for readability reasons), you will have to escape the delimiter
if you want to match it. Perl won't warn then. See also "Gory details of parsing quoted con-structs" constructs"
structs" in perlop.
All the sequences and escapes
\000 Octal escape sequence.
\1 Absolute backreference.
\a Alarm or bell.
\A Beginning of string.
\b Word/non-word boundary. (Backspace in a char class).
\B Not a word/non-word boundary.
\cX Control-X (X can be any ASCII character).
\C Single octet, even under UTF-8.
\d Character class for digits.
\D Character class for non-digits.
\e Escape character.
\E Turn off \Q, \L and \U processing.
\f Form feed.
\G Pos assertion.
\l Lowercase next character.
\L Lowercase till \E.
\n (Logical) newline character.
\N{} Named (Unicode) character.
\p{}, \pP Character with a Unicode property.
\P{}, \PP Character without a Unicode property.
\Q Quotemeta till \E.
\r Return character.
\s Character class for white space.
\S Character class for non white space.
\t Tab character.
\u Titlecase next character.
\U Uppercase till \E.
\w Character class for word characters.
\W Character class for non-word characters.
\x{}, \x00 Hexadecimal escape sequence.
\X Extended Unicode "combining character sequence".
\z End of string.
\Z End of string.
Character Escapes
Fixed characters
A handful of characters have a dedicated character escape. The following table shows them, along with
their code points (in decimal and hex), their ASCII name, the control escape (see below) and a short
description.
Seq. Code Point ASCII Cntr Description.
Dec Hex
\a 7 07 BEL \cG alarm or bell
\b 8 08 BS \cH backspace [1]
\e 27 1B ESC \c[ escape character
\f 12 0C FF \cL form feed
\n 10 0A LF \cJ line feed [2]
\r 13 0D CR \cM carriage return
\t 9 09 TAB \cI tab
[1] "\b" is only the backspace character inside a character class. Outside a character class, "\b" is
a word/non-word boundary.
[2] "\n" matches a logical newline. Perl will convert between "\n" and your OSses native newline
character when reading from or writing to text files.
Example
$str =~ /\t/; # Matches if $str contains a (horizontal) tab.
Control characters
"\c" is used to denote a control character; the character following "\c" is the name of the control
character. For instance, "/\cM/" matches the character control-M (a carriage return, code point 13).
The case of the character following "\c" doesn't matter: "\cM" and "\cm" match the same character.
Mnemonic: control character.
Example
$str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
Named characters
All Unicode characters have a Unicode name, and characters in various scripts have names as well. It
is even possible to give your own names to characters. You can use a character by name by using the
"\N{}" construct; the name of the character goes between the curly braces. You do have to "use char-names" charnames"
names" to load the names of the characters, otherwise Perl will complain you use a name it doesn't
know about. For more details, see charnames.
Mnemonic: Named character.
Example
use charnames ':full'; # Loads the Unicode names.
$str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
use charnames 'Cyrillic'; # Loads Cyrillic names.
$str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
Octal escapes
Octal escapes consist of a backslash followed by two or three octal digits matching the code point of
the character you want to use. This allows for 512 characters ("\00" up to "\777") that can be
expressed this way. Enough in pre-Unicode days, but most Unicode characters cannot be escaped this
way.
Note that a character that is expressed as an octal escape is considered as a character without spe-cial special
cial meaning by the regex engine, and will match "as is".
Examples
$str = "Perl";
$str =~ /\120/; # Match, "\120" is "P".
$str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
$str =~ /P\053/; # No match, "\053" is "+" and taken literally.
Caveat
Octal escapes potentially clash with backreferences. They both consist of a backslash followed by
numbers. So Perl has to use heuristics to determine whether it is a backreference or an octal escape.
Perl uses the following rules:
1 If the backslash is followed by a single digit, it's a backreference.
2 If the first digit following the backslash is a 0, it's an octal escape.
3 If the number following the backslash is N (decimal), and Perl already has seen N capture groups,
Perl will consider this to be a backreference. Otherwise, it will consider it to be an octal
escape. Note that if N > 999, Perl only takes the first three digits for the octal escape; the
rest is matched as is.
my $pat = "(" x 999;
$pat .= "a";
$pat .= ")" x 999;
/^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
/^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
# and \1000 is seen as \100 (a '@') and a '0'.
Hexadecimal escapes
Hexadecimal escapes start with "\x" and are then either followed by two digit hexadecimal number, or
a hexadecimal number of arbitrary length surrounded by curly braces. The hexadecimal number is the
code point of the character you want to express.
Note that a character that is expressed as a hexadecimal escape is considered as a character without
special meaning by the regex engine, and will match "as is".
Mnemonic: hexadecimal.
Examples
$str = "Perl";
$str =~ /\x50/; # Match, "\x50" is "P".
$str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
$str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
/\x{2603}\x{2602}/ # Snowman with an umbrella.
# The Unicode character 2603 is a snowman,
# the Unicode character 2602 is an umbrella.
/\x{263B}/ # Black smiling face.
/\x{263b}/ # Same, the hex digits A - F are case insensitive.
Modifiers
A number of backslash sequences have to do with changing the character, or characters following them.
"\l" will lowercase the character following it, while "\u" will uppercase (or, more accurately,
titlecase) the character following it. (They perform similar functionality as the functions "lcfirst"
and "ucfirst").
To uppercase or lowercase several characters, one might want to use "\L" or "\U", which will lower-case/uppercase lowercase/uppercase
case/uppercase all characters following them, until either the end of the pattern, or the next occur-rence occurrence
rence of "\E", whatever comes first. They perform similar functionality as the functions "lc" and
"uc" do.
"\Q" is used to escape all characters following, up to the next "\E" or the end of the pattern. "\Q"
adds a backslash to any character that isn't a letter, digit or underscore. This will ensure that any
character between "\Q" and "\E" is matched literally, and will not be interpreted by the regexp
engine.
Mnemonic: Lowercase, Uppercase, Quotemeta, End.
Examples
$sid = "sid";
$greg = "GrEg";
$miranda = "(Miranda)";
$str =~ /\u$sid/; # Matches 'Sid'
$str =~ /\L$greg/; # Matches 'greg'
$str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
# had been written as /\(Miranda\)/
Character classes
Perl regular expressions have a large range of character classes. Some of the character classes are
written as a backslash sequence. We will briefly discuss those here; full details of character
classes can be found in perlrecharclass.
"\w" is a character class that matches any word character (letters, digits, underscore). "\d" is a
character class that matches any digit, while the character class "\s" matches any white space char-acter. character.
acter.
The uppercase variants ("\W", "\D", "\S") are character classes that match any character that isn't a
word character, digit or white space.
Mnemonics: word, digit, space
Unicode classes
"\pP" (where "P" is a single letter) and "\p{Property}" are used to match a character that matches
the given Unicode property; properties include things like "letter", or "thai character". Capitaliz-ing Capitalizing
ing the sequence to "\PP" and "\P{Property}" make the sequence match a character that doesn't match
the given Unicode property. For more details, see "Backslashed sequences" in perlrecharclass and
"Unicode Character Properties" in perlunicode.
Mnemonic: property.
Referencing
If capturing parenthesis are used in a regular expression, we can refer to the part of the source
string that was matched, and match exactly the same thing. In Perl 5.8.x and earlier there is only
one way of referring to a backreference, by absolution number. Perl 5.10 adds the ability to refer-ence reference
ence relatively and by name.
Absolute referencing
A backslash sequence that starts with a backslash and is followed by a number is an absolute refer-ence reference
ence (but be aware of the caveat mentioned above). If the number is N, it refers to the Nth set of
parenthesis - whatever has been matched by that set of parenthesis has to be matched by the "\N" as
well.
Examples
/(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
/(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
Assertions
Assertions are conditions that have to be true -- they don't actually match parts of the substring.
There are six assertions that are written as backslash sequences.
\A "\A" only matches at the beginning of the string. If the "/m" modifier isn't used, then "/\A/" is
equivalent with "/^/". However, if the "/m" modifier is used, then "/^/" matches internal new-lines, newlines,
lines, but the meaning of "/\A/" isn't changed by the "/m" modifier. "\A" matches at the begin-ning beginning
ning of the string regardless whether the "/m" modifier is used.
\z, \Z
"\z" and "\Z" match at the end of the string. If the "/m" modifier isn't used, then "/\Z/" is
equivalent with "/$/", that is, it matches at the end of the string, or before the newline at the
end of the string. If the "/m" modifier is used, then "/$/" matches at internal newlines, but the
meaning of "/\Z/" isn't changed by the "/m" modifier. "\Z" matches at the end of the string (or
just before a trailing newline) regardless whether the "/m" modifier is used.
"\z" is just like "\Z", except that it will not match before a trailing newline. "\z" will only
match at the end of the string - regardless of the modifiers used, and not before a newline.
\G "\G" is usually only used in combination with the "/g" modifier. If the "/g" modifier is used
(and the match is done in scalar context), Perl will remember where in the source string the last
match ended, and the next time, it will start the match from where it ended the previous time.
"\G" matches the point where the previous match ended, or the beginning of the string if there
was no previous match.
Mnemonic: Global.
\b, \B
"\b" matches at any place between a word and a non-word character; "\B" matches at any place
between characters where "\b" doesn't match. "\b" and "\B" assume there's a non-word character
before the beginning and after the end of the source string; so "\b" will match at the beginning
(or end) of the source string if the source string begins (or ends) with a word character. Other-wise, Otherwise,
wise, "\B" will match.
Mnemonic: boundary.
Examples
"cat" =~ /\Acat/; # Match.
"cat" =~ /cat\Z/; # Match.
"cat\n" =~ /cat\Z/; # Match.
"cat\n" =~ /cat\z/; # No match.
"cat" =~ /\bcat\b/; # Matches.
"cats" =~ /\bcat\b/; # No match.
"cat" =~ /\bcat\B/; # No match.
"cats" =~ /\bcat\B/; # Match.
while ("cat dog" =~ /(\w+)/g) {
print $1; # Prints 'catdog'
}
while ("cat dog" =~ /\G(\w+)/g) {
print $1; # Prints 'cat'
}
Misc
Here we document the backslash sequences that don't fall in one of the categories above. They are:
\C "\C" always matches a single octet, even if the source string is encoded in UTF-8 format, and the
character to be matched is a multi-octet character. "\C" was introduced in perl 5.6.
Mnemonic: oCtet.
\X This matches an extended Unicode combining character sequence, and is equivalent to
"(?>\PM\pM*)". "\PM" matches any character that is not considered a Unicode mark character, while
"\pM" matches any character that is considered a Unicode mark character; so "\X" matches any non
mark character followed by zero or more mark characters. Mark characters include (but are not
restricted to) combining characters and vowel signs.
"\X" matches quite well what normal (non-Unicode-programmer) usage would consider a single char-acter: character:
acter: for example a base character (the "\PM" above), for example a letter, followed by zero or
more diacritics, which are combining characters (the "\pM*" above).
Mnemonic: eXtended Unicode character.
Examples
"\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
$str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
$str =~ s/(.)\K\1//g; # Delete duplicated characters.
"\n" =~ /^\R$/; # Match, \n is a generic newline.
"\r" =~ /^\R$/; # Match, \r is a generic newline.
"\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
"P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
perl v5.8.9 2007-11-17 PERLREBACKSLASH(1)
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