PERLRECHARCLASS(1) Perl Programmers Reference Guide PERLRECHARCLASS(1)NAMEperlrecharclass - 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 remem‐
ber that matching a character class consumes exactly one character in
the source string. (The source string is the string the regular expres‐
sion 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 charac‐
ter, except for the newline. The default can be changed to add matching
the newline with the single line modifier: either for the entire regu‐
lar 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 con‐
stitute 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.
\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 charac‐
ters 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. With‐
out a locale, "\s" matches the five characters mentioned in the begin‐
ning 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.
It is worth noting that "\d", "\w", etc, match single characters, not
complete numbers or words. To match a number (that consists of inte‐
gers), 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{LowercaseLet‐
ter}/". "/\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 perluni‐
code.
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.
"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 expres‐
sions is the bracketed form. In its simplest form, it lists the charac‐
ters that may be matched inside square brackets, like this: "[aeiou]".
This matches one of "a", "e", "i", "o" or "u". Just as the other char‐
acter classes, exactly one character will be matched. To match a longer
string consisting of characters mentioned in the characters class, fol‐
low 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 consid‐
ered 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 char‐
acter 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 charac‐
ters 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 escap‐
ing.
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. How‐
ever, 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 consid‐
ered part of the set of characters that can be matched without escap‐
ing.
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 char‐
acters 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 nec‐
essary 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 char‐
acter 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 let‐
ters 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 prop‐
erties, 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 exam‐
ple newline and backspace are control characters. All characters
with "ord()" less than 32 are usually classified as control charac‐
ters (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 con‐
sists 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 v5.8.8 2008-09-19 PERLRECHARCLASS(1)
