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perlrecharclass - Perl Regular Expression Character Classes
The top level documentation about Perl regular expressions
is found in the perlre manpage.
This manual page discusses the syntax and use of character
classes in Perl Regular Expressions.
A character class is a way of denoting a set of characters,
in such a way that one character of the set is matched.
It's important to remember that matching a character class
consumes exactly one character in the source string. (The source
string is the string the regular expression is matched against.)
There are three types of character classes in Perl regular
expressions: the dot, backslashed sequences, and the bracketed form.
The dot (or period), . is probably the most used, and certainly
the most well-known character class. By default, a dot matches any
character, except for the newline. The default can be changed to
add matching the newline with the single line modifier: either
for the entire regular expression using the /s modifier, or
locally using (?s).
Here are some examples:
"a" =~ /./ # Match
"." =~ /./ # Match
"" =~ /./ # No match (dot has to match a character)
"\n" =~ /./ # No match (dot does not match a newline)
"\n" =~ /./s # Match (global 'single line' modifier)
"\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
"ab" =~ /^.$/ # No match (dot matches one character)
Perl regular expressions contain many backslashed sequences that
constitute a character class. That is, they will match a single
character, if that character belongs to a specific set of characters
(defined by the sequence). A backslashed sequence is a sequence of
characters starting with a backslash. Not all backslashed sequences
are character class; for a full list, see the perlrebackslash manpage.
Here's a list of the backslashed sequences, which are discussed in
more detail below.
\d Match a digit character.
\D Match a non-digit character.
\w Match a "word" character.
\W Match a non-"word" character.
\s Match a white space character.
\S Match a non-white space character.
\h Match a horizontal white space character.
\H Match a character that isn't horizontal white space.
\v Match a vertical white space character.
\V Match a character that isn't vertical white space.
\pP, \p{Prop} Match a character matching a Unicode property.
\PP, \P{Prop} Match a character that doesn't match a Unicode property.
\d matches a single character that is considered to be a digit.
What is considered a digit depends on the internal encoding of
the source string. If the source string is in UTF-8 format, \d
not only matches the digits '0' - '9', but also Arabic, Devanagari and
digits from other languages. Otherwise, if there is a locale in effect,
it will match whatever characters the locale considers digits. Without
a locale, \d matches the digits '0' to '9'.
See Locale, Unicode and UTF-8.
Any character that isn't matched by \d will be matched by \D.
\w matches a single word character: an alphanumeric character
(that is, an alphabetic character, or a digit), or the underscore (_).
What is considered a word character depends on the internal encoding
of the string. If it's in UTF-8 format, \w matches those characters
that are considered word characters in the Unicode database. That is, it
not only matches ASCII letters, but also Thai letters, Greek letters, etc.
If the source string isn't in UTF-8 format, \w matches those characters
that are considered word characters by the current locale. Without
a locale in effect, \w matches the ASCII letters, digits and the
underscore.
Any character that isn't matched by \w will be matched by \W.
\s matches any single character that is consider white space. In the
ASCII range, \s matches the horizontal tab (\t), the new line
(\n), the form feed (\f), the carriage return (\r), and the
space (the vertical tab, \cK is not matched by \s). The exact set
of characters matched by \s depends on whether the source string is
in UTF-8 format. If it is, \s matches what is considered white space
in the Unicode database. Otherwise, if there is a locale in effect, \s
matches whatever is considered white space by the current locale. Without
a locale, \s matches the five characters mentioned in the beginning
of this paragraph. Perhaps the most notable difference is that \s
matches a non-breaking space only if the non-breaking space is in a
UTF-8 encoded string.
Any character that isn't matched by \s will be matched by \S.
\h will match any character that is considered horizontal white space;
this includes the space and the tab characters. \H will match any character
that is not considered horizontal white space.
\v will match any character that is considered vertical white space;
this includes the carriage return and line feed characters (newline).
\V will match any character that is not considered vertical white space.
\R matches anything that can be considered a newline under Unicode
rules. It's not a character class, as it can match a multi-character
sequence. Therefore, it cannot be used inside a bracketed character
class. Details are discussed in the perlrebackslash manpage.
\h, \H, \v, \V, and \R are new in perl 5.10.0.
Note that unlike \s, \d and \w, \h and \v always match
the same characters, regardless whether the source string is in UTF-8
format or not. The set of characters they match is also not influenced
by locale.
One might think that \s is equivalent with [\h\v]. This is not true.
The vertical tab ("\x0b") is not matched by \s, it is however
considered vertical white space. Furthermore, if the source string is
not in UTF-8 format, the next line ("\x85") and the no-break space
("\xA0") are not matched by \s, but are by \v and \h respectively.
If the source string is in UTF-8 format, both the next line and the
no-break space are matched by \s.
The following table is a complete listing of characters matched by
\s, \h and \v.
The first column gives the code point of the character (in hex format),
the second column gives the (Unicode) name. The third column indicates
by which class(es) the character is matched.
0x00009 CHARACTER TABULATION h s
0x0000a LINE FEED (LF) vs
0x0000b LINE TABULATION v
0x0000c FORM FEED (FF) vs
0x0000d CARRIAGE RETURN (CR) vs
0x00020 SPACE h s
0x00085 NEXT LINE (NEL) vs [1]
0x000a0 NO-BREAK SPACE h s [1]
0x01680 OGHAM SPACE MARK h s
0x0180e MONGOLIAN VOWEL SEPARATOR h s
0x02000 EN QUAD h s
0x02001 EM QUAD h s
0x02002 EN SPACE h s
0x02003 EM SPACE h s
0x02004 THREE-PER-EM SPACE h s
0x02005 FOUR-PER-EM SPACE h s
0x02006 SIX-PER-EM SPACE h s
0x02007 FIGURE SPACE h s
0x02008 PUNCTUATION SPACE h s
0x02009 THIN SPACE h s
0x0200a HAIR SPACE h s
0x02028 LINE SEPARATOR vs
0x02029 PARAGRAPH SEPARATOR vs
0x0202f NARROW NO-BREAK SPACE h s
0x0205f MEDIUM MATHEMATICAL SPACE h s
0x03000 IDEOGRAPHIC SPACE h s
- [1]
-
NEXT LINE and NO-BREAK SPACE only match \s if the source string is in
UTF-8 format.
It is worth noting that \d, \w, etc, match single characters, not
complete numbers or words. To match a number (that consists of integers),
use \d+; to match a word, use \w+.
\pP and \p{Prop} are character classes to match characters that
fit given Unicode classes. One letter classes can be used in the \pP
form, with the class name following the \p, otherwise, the property
name is enclosed in braces, and follows the \p. For instance, a
match for a number can be written as /\pN/ or as /\p{Number}/.
Lowercase letters are matched by the property LowercaseLetter which
has as short form Ll. They have to be written as /\p{Ll}/ or
/\p{LowercaseLetter}/. /\pLl/ is valid, but means something different.
It matches a two character string: a letter (Unicode property \pL),
followed by a lowercase l.
For a list of possible properties, see
Unicode Character Properties in the perlunicode manpage. It is also possible to
defined your own properties. This is discussed in
User-Defined Character Properties in the perlunicode manpage.
"a" =~ /\w/ # Match, "a" is a 'word' character.
"7" =~ /\w/ # Match, "7" is a 'word' character as well.
"a" =~ /\d/ # No match, "a" isn't a digit.
"7" =~ /\d/ # Match, "7" is a digit.
" " =~ /\s/ # Match, a space is white space.
"a" =~ /\D/ # Match, "a" is a non-digit.
"7" =~ /\D/ # No match, "7" is not a non-digit.
" " =~ /\S/ # No match, a space is not non-white space.
" " =~ /\h/ # Match, space is horizontal white space.
" " =~ /\v/ # No match, space is not vertical white space.
"\r" =~ /\v/ # Match, a return is vertical white space.
"a" =~ /\pL/
"a" =~ /\p{Lu}/
"\x{0e0b}" =~ /\p{Thai}/
"a" =~ /\P{Lao}/
The third form of character class you can use in Perl regular expressions
is the bracketed form. In its simplest form, it lists the characters
that may be matched inside square brackets, like this: [aeiou].
This matches one of a, e, i, o or u. Just as the other
character classes, exactly one character will be matched. To match
a longer string consisting of characters mentioned in the characters
class, follow the character class with a quantifier. For instance,
[aeiou]+ matches a string of one or more lowercase ASCII vowels.
Repeating a character in a character class has no
effect; it's considered to be in the set only once.
Examples:
"e" =~ /[aeiou]/ # Match, as "e" is listed in the class.
"p" =~ /[aeiou]/ # No match, "p" is not listed in the class.
"ae" =~ /^[aeiou]$/ # No match, a character class only matches
# a single character.
"ae" =~ /^[aeiou]+$/ # Match, due to the quantifier.
Most characters that are meta characters in regular expressions (that
is, characters that carry a special meaning like * or () lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, [()] matches either an opening
parenthesis, or a closing parenthesis, and the parens inside the character
class don't group or capture.
Characters that may carry a special meaning inside a character class are:
\, ^, -, [ and ], and are discussed below. They can be
escaped with a backslash, although this is sometimes not needed, in which
case the backslash may be omitted.
The sequence \b is special inside a bracketed character class. While
outside the character class \b is an assertion indicating a point
that does not have either two word characters or two non-word characters
on either side, inside a bracketed character class, \b matches a
backspace character.
A [ is not special inside a character class, unless it's the start
of a POSIX character class (see below). It normally does not need escaping.
A ] is either the end of a POSIX character class (see below), or it
signals the end of the bracketed character class. Normally it needs
escaping if you want to include a ] in the set of characters.
However, if the ] is the first (or the second if the first
character is a caret) character of a bracketed character class, it
does not denote the end of the class (as you cannot have an empty class)
and is considered part of the set of characters that can be matched without
escaping.
Examples:
"+" =~ /[+?*]/ # Match, "+" in a character class is not special.
"\cH" =~ /[\b]/ # Match, \b inside in a character class
# is equivalent with a backspace.
"]" =~ /[][]/ # Match, as the character class contains.
# both [ and ].
"[]" =~ /[[]]/ # Match, the pattern contains a character class
# containing just ], and the character class is
# followed by a ].
It is not uncommon to want to match a range of characters. Luckily, instead
of listing all the characters in the range, one may use the hyphen (-).
If inside a bracketed character class you have two characters separated
by a hyphen, it's treated as if all the characters between the two are in
the class. For instance, [0-9] matches any ASCII digit, and [a-m]
matches any lowercase letter from the first half of the ASCII alphabet.
Note that the two characters on either side of the hyphen are not
necessary both letters or both digits. Any character is possible,
although not advisable. ['-?] contains a range of characters, but
most people will not know which characters that will be. Furthermore,
such ranges may lead to portability problems if the code has to run on
a platform that uses a different character set, such as EBCDIC.
If a hyphen in a character class cannot be part of a range, for instance
because it is the first or the last character of the character class,
or if it immediately follows a range, the hyphen isn't special, and will be
considered a character that may be matched. You have to escape the hyphen
with a backslash if you want to have a hyphen in your set of characters to
be matched, and its position in the class is such that it can be considered
part of a range.
Examples:
[a-z]
[a-fz]
[-z]
[a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
# hyphen ('-'), or the letter 'm'.
['-?]
It is also possible to instead list the characters you do not want to
match. You can do so by using a caret (^) as the first character in the
character class. For instance, [^a-z] matches a character that is not a
lowercase ASCII letter.
This syntax make the caret a special character inside a bracketed character
class, but only if it is the first character of the class. So if you want
to have the caret as one of the characters you want to match, you either
have to escape the caret, or not list it first.
Examples:
"e" =~ /[^aeiou]/ # No match, the 'e' is listed.
"x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel.
"^" =~ /[^^]/ # No match, matches anything that isn't a caret.
"^" =~ /[x^]/ # Match, caret is not special here.
You can put a backslash sequence character class inside a bracketed character
class, and it will act just as if you put all the characters matched by
the backslash sequence inside the character class. For instance,
[a-f\d] will match any digit, or any of the lowercase letters between
'a' and 'f' inclusive.
Examples:
/[\p{Thai}\d]/
/[^\p{Arabic}()]/
Backslash sequence character classes cannot form one of the endpoints
of a range.
Posix character classes have the form [:class:], where class is
name, and the [: and :] delimiters. Posix character classes appear
inside bracketed character classes, and are a convenient and descriptive
way of listing a group of characters. Be careful about the syntax,
# Correct:
$string =~ /[[:alpha:]]/
# Incorrect (will warn):
$string =~ /[:alpha:]/
The latter pattern would be a character class consisting of a colon,
and the letters a, l, p and h.
Perl recognizes the following POSIX character classes:
alpha Any alphabetical character.
alnum Any alphanumerical character.
ascii Any ASCII character.
blank A GNU extension, equal to a space or a horizontal tab (C<\t>).
cntrl Any control character.
digit Any digit, equivalent to C<\d>.
graph Any printable character, excluding a space.
lower Any lowercase character.
print Any printable character, including a space.
punct Any punctuation character.
space Any white space cha | 
