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PERLRECHARCLASS(1) Perl Programmers Reference Guide PERLRECHARCLASS(1)
NAME
perlrecharclass - Perl Regular Expression Character Classes
DESCRIPTION
The top level documentation about Perl regular expressions is found in perlre.
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
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)
Backslashed sequences
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 perlrebackslash.
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.
Digits
"\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".
Word characters
"\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".
White space
"\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 perlrebackslash.
"\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+".
Unicode Properties
"\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 perlunicode. It
is also possible to defined your own properties. This is discussed in "User-Defined
Character Properties" in perlunicode.
Examples
"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/ # Match, "a" is a letter.
"a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters.
"\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character
# 'THAI CHARACTER SO SO', and that's in
# Thai Unicode class.
"a" =~ /\P{Lao}/ # Match, as "a" is not a Laoian character.
Bracketed Character Classes
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.
Special Characters Inside a Bracketed Character Class
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 ].
Character Ranges
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] # Matches a character that is a lower case ASCII letter.
[a-fz] # Matches any letter between 'a' and 'f' (inclusive) or the
# letter 'z'.
[-z] # Matches either a hyphen ('-') or the letter 'z'.
[a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
# hyphen ('-'), or the letter 'm'.
['-?] # Matches any of the characters '()*+,-./0123456789:;<=>?
# (But not on an EBCDIC platform).
Negation
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.
Backslash Sequences
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]/ # Matches a character that is either a Thai
# character, or a digit.
/[^\p{Arabic}()]/ # Matches a character that is neither an Arabic
# character, nor a parenthesis.
Backslash sequence character classes cannot form one of the endpoints of a range.
Posix Character Classes
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 character. C<\s> plus the vertical tab (C<\cK>).
upper Any uppercase character.
word Any "word" character, equivalent to C<\w>.
xdigit Any hexadecimal digit, '0' - '9', 'a' - 'f', 'A' - 'F'.
The exact set of characters matched depends on whether the source string is internally in
UTF-8 format or not. See "Locale, Unicode and UTF-8".
Most POSIX character classes have "\p" counterparts. The difference is that the "\p"
classes will always match according to the Unicode properties, regardless whether the
string is in UTF-8 format or not.
The following table shows the relation between POSIX character classes and the Unicode
properties:
[[:...:]] \p{...} backslash
alpha IsAlpha
alnum IsAlnum
ascii IsASCII
blank
cntrl IsCntrl
digit IsDigit \d
graph IsGraph
lower IsLower
print IsPrint
punct IsPunct
space IsSpace
IsSpacePerl \s
upper IsUpper
word IsWord
xdigit IsXDigit
Some character classes may have a non-obvious name:
cntrl
Any control character. Usually, control characters don't produce output as such, but
instead control the terminal somehow: for example newline and backspace are control
characters. All characters with "ord()" less than 32 are usually classified as control
characters (in ASCII, the ISO Latin character sets, and Unicode), as is the character
"ord()" value of 127 ("DEL").
graph
Any character that is graphical, that is, visible. This class consists of all the
alphanumerical characters and all punctuation characters.
print
All printable characters, which is the set of all the graphical characters plus the
space.
punct
Any punctuation (special) character.
Negation
A Perl extension to the POSIX character class is the ability to negate it. This is done by
prefixing the class name with a caret ("^"). Some examples:
POSIX Unicode Backslash
[[:^digit:]] \P{IsDigit} \D
[[:^space:]] \P{IsSpace} \S
[[:^word:]] \P{IsWord} \W
[= =] and [. .]
Perl will recognize the POSIX character classes "[=class=]", and "[.class.]", but does not
(yet?) support this construct. Use of such a constructs will lead to an error.
Examples
/[[:digit:]]/ # Matches a character that is a digit.
/[01[:lower:]]/ # Matches a character that is either a
# lowercase letter, or '0' or '1'.
/[[:digit:][:^xdigit:]]/ # Matches a character that can be anything,
# but the letters 'a' to 'f' in either case.
# This is because the character class contains
# all digits, and anything that isn't a
# hex digit, resulting in a class containing
# all characters, but the letters 'a' to 'f'
# and 'A' to 'F'.
Locale, Unicode and UTF-8
Some of the character classes have a somewhat different behaviour depending on the
internal encoding of the source string, and the locale that is in effect.
"\w", "\d", "\s" and the POSIX character classes (and their negations, including "\W",
"\D", "\S") suffer from this behaviour.
The rule is that if the source string is in UTF-8 format, the character classes match
according to the Unicode properties. If the source string isn't, then the character
classes match according to whatever locale is in effect. If there is no locale, they match
the ASCII defaults (52 letters, 10 digits and underscore for "\w", 0 to 9 for "\d", etc).
This usually means that if you are matching against characters whose "ord()" values are
between 128 and 255 inclusive, your character class may match or not depending on the
current locale, and whether the source string is in UTF-8 format. The string will be in
UTF-8 format if it contains characters whose "ord()" value exceeds 255. But a string may
be in UTF-8 format without it having such characters.
For portability reasons, it may be better to not use "\w", "\d", "\s" or the POSIX
character classes, and use the Unicode properties instead.
Examples
$str = "\xDF"; # $str is not in UTF-8 format.
$str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
$str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
$str =~ /^\w/; # Match! $str is now in UTF-8 format.
chop $str;
$str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
perl v5.10.0 2011-06-30 PERLRECHARCLASS(1)
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