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,
backslash sequences, and the form enclosed in square brackets. Keep in mind,
though, that often the term "character class" is used to mean just
the bracketed form. Certainly, most Perl documentation does that.
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. That default can be changed to add matching the
newline by using the
single line modifier: either for the entire
regular expression with the "/s" modifier, or locally with
"(?s)". (The "\N" backslash sequence, described below,
matches any character except newline without regard to the
single line
modifier.)
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)
Backslash sequences¶
A backslash sequence is a sequence of characters, the first one of which is a
backslash. Perl ascribes special meaning to many such sequences, and some of
these are character classes. That is, they match a single character each,
provided that the character belongs to the specific set of characters defined
by the sequence.
Here's a list of the backslash sequences that are character classes. They are
discussed in more detail below. (For the backslash sequences that aren't
character classes, see perlrebackslash.)
\d Match a decimal digit character.
\D Match a non-decimal-digit character.
\w Match a "word" character.
\W Match a non-"word" character.
\s Match a whitespace character.
\S Match a non-whitespace character.
\h Match a horizontal whitespace character.
\H Match a character that isn't horizontal whitespace.
\v Match a vertical whitespace character.
\V Match a character that isn't vertical whitespace.
\N Match a character that isn't a newline.
\pP, \p{Prop} Match a character that has the given Unicode property.
\PP, \P{Prop} Match a character that doesn't have the Unicode property
\N
"\N", available starting in v5.12, like the dot, matches any character
that is not a newline. The difference is that "\N" is not influenced
by the
single line regular expression modifier (see "The dot"
above). Note that the form "\N{...}" may mean something completely
different. When the "{...}" is a quantifier, it means to match a
non-newline character that many times. For example, "\N{3}" means to
match 3 non-newlines; "\N{5,}" means to match 5 or more
non-newlines. But if "{...}" is not a legal quantifier, it is
presumed to be a named character. See charnames for those. For example, none
of "\N{COLON}", "\N{4F}", and "\N{F4}" contain
legal quantifiers, so Perl will try to find characters whose names are
respectively "COLON", "4F", and "F4".
Digits
"\d" matches a single character considered to be a decimal
digit. If the "/a" regular expression modifier is in effect,
it matches [0-9]. Otherwise, it matches anything that is matched by
"\p{Digit}", which includes [0-9]. (An unlikely possible exception
is that under locale matching rules, the current locale might not have
"[0-9]" matched by "\d", and/or might match other
characters whose code point is less than 256. The only such locale definitions
that are legal would be to match "[0-9]" plus another set of 10
consecutive digit characters; anything else would be in violation of the C
language standard, but Perl doesn't currently assume anything in regard to
this.)
What this means is that unless the "/a" modifier is in effect
"\d" not only matches the digits '0' - '9', but also Arabic,
Devanagari, and digits from other languages. This may cause some confusion,
and some security issues.
Some digits that "\d" matches look like some of the [0-9] ones, but
have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks very
much like an ASCII DIGIT EIGHT (U+0038). An application that is expecting only
the ASCII digits might be misled, or if the match is "\d+", the
matched string might contain a mixture of digits from different writing
systems that look like they signify a number different than they actually do.
"
num()" in Unicode::UCD can be used to safely calculate the
value, returning "undef" if the input string contains such a
mixture.
What "\p{Digit}" means (and hence "\d" except under the
"/a" modifier) is "\p{General_Category=Decimal_Number}",
or synonymously, "\p{General_Category=Digit}". Starting with Unicode
version 4.1, this is the same set of characters matched by
"\p{Numeric_Type=Decimal}". But Unicode also has a different
property with a similar name, "\p{Numeric_Type=Digit}", which
matches a completely different set of characters. These characters are things
such as "CIRCLED DIGIT ONE" or subscripts, or are from writing
systems that lack all ten digits.
The design intent is for "\d" to exactly match the set of characters
that can safely be used with "normal" big-endian positional decimal
syntax, where, for example 123 means one 'hundred', plus two 'tens', plus
three 'ones'. This positional notation does not necessarily apply to
characters that match the other type of "digit",
"\p{Numeric_Type=Digit}", and so "\d" doesn't match them.
The Tamil digits (U+0BE6 - U+0BEF) can also legally be used in old-style Tamil
numbers in which they would appear no more than one in a row, separated by
characters that mean "times 10", "times 100", etc. (See
<
http://www.unicode.org/notes/tn21>.)
Any character not matched by "\d" is matched by "\D".
Word characters
A "\w" matches a single alphanumeric character (an alphabetic
character, or a decimal digit); or a connecting punctuation character, such as
an underscore ("_"); or a "mark" character (like some sort
of accent) that attaches to one of those. It does not match a whole word. To
match a whole word, use "\w+". This isn't the same thing as matching
an English word, but in the ASCII range it is the same as a string of
Perl-identifier characters.
- If the "/a" modifier is in effect ...
- "\w" matches the 63 characters [a-zA-Z0-9_].
- otherwise ...
- For code points above 255 ...
- "\w" matches the same as "\p{Word}" matches in this
range. That is, it matches Thai letters, Greek letters, etc. This includes
connector punctuation (like the underscore) which connect two words
together, or diacritics, such as a "COMBINING TILDE" and the
modifier letters, which are generally used to add auxiliary markings to
letters.
- For code points below 256 ...
- if locale rules are in effect ...
- "\w" matches the platform's native underscore character plus
whatever the locale considers to be alphanumeric.
- if Unicode rules are in effect ...
- "\w" matches exactly what "\p{Word}" matches.
- otherwise ...
- "\w" matches [a-zA-Z0-9_].
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
There are a number of security issues with the full Unicode list of word
characters. See <
http://unicode.org/reports/tr36>.
Also, for a somewhat finer-grained set of characters that are in programming
language identifiers beyond the ASCII range, you may wish to instead use the
more customized "Unicode Properties", "\p{ID_Start}",
"\p{ID_Continue}", "\p{XID_Start}", and
"\p{XID_Continue}". See <
http://unicode.org/reports/tr31>.
Any character not matched by "\w" is matched by "\W".
Whitespace
"\s" matches any single character considered whitespace.
- If the "/a" modifier is in effect ...
- In all Perl versions, "\s" matches the 5 characters [\t\n\f\r ];
that is, the horizontal tab, the newline, the form feed, the carriage
return, and the space. Starting in Perl v5.18, experimentally, it also
matches the vertical tab, "\cK". See note "[1]" below
for a discussion of this.
- otherwise ...
- For code points above 255 ...
- "\s" matches exactly the code points above 255 shown with an
"s" column in the table below.
- For code points below 256 ...
- if locale rules are in effect ...
- "\s" matches whatever the locale considers to be
whitespace.
- if Unicode rules are in effect ...
- "\s" matches exactly the characters shown with an "s"
column in the table below.
- otherwise ...
- "\s" matches [\t\n\f\r ] and, starting, experimentally in Perl
v5.18, the vertical tab, "\cK". (See note "[1]" below
for a discussion of this.) Note that this list doesn't include the
non-breaking space.
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
Any character not matched by "\s" is matched by "\S".
"\h" matches any character considered horizontal whitespace; this
includes the platform's space and tab characters and several others listed in
the table below. "\H" matches any character not considered
horizontal whitespace. They use the platform's native character set, and do
not consider any locale that may otherwise be in use.
"\v" matches any character considered vertical whitespace; this
includes the platform's carriage return and line feed characters (newline)
plus several other characters, all listed in the table below. "\V"
matches any character not considered vertical whitespace. They use the
platform's native character set, and do not consider any locale that may
otherwise be in use.
"\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; use
"\v" instead (vertical whitespace). It uses the platform's native
character set, and does not consider any locale that may otherwise be in use.
Details are discussed in perlrebackslash.
Note that unlike "\s" (and "\d" and "\w"),
"\h" and "\v" always match the same characters, without
regard to other factors, such as the active locale or whether the source
string is in UTF-8 format.
One might think that "\s" is equivalent to "[\h\v]". This is
indeed true starting in Perl v5.18, but prior to that, the sole difference was
that the vertical tab ("\cK") was not matched by "\s".
The following table is a complete listing of characters matched by
"\s", "\h" and "\v" as of Unicode 6.3.
The first column gives the Unicode 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 (assuming no locale is in effect that
changes the "\s" matching).
0x0009 CHARACTER TABULATION h s
0x000a LINE FEED (LF) vs
0x000b LINE TABULATION vs [1]
0x000c FORM FEED (FF) vs
0x000d CARRIAGE RETURN (CR) vs
0x0020 SPACE h s
0x0085 NEXT LINE (NEL) vs [2]
0x00a0 NO-BREAK SPACE h s [2]
0x1680 OGHAM SPACE MARK h s
0x2000 EN QUAD h s
0x2001 EM QUAD h s
0x2002 EN SPACE h s
0x2003 EM SPACE h s
0x2004 THREE-PER-EM SPACE h s
0x2005 FOUR-PER-EM SPACE h s
0x2006 SIX-PER-EM SPACE h s
0x2007 FIGURE SPACE h s
0x2008 PUNCTUATION SPACE h s
0x2009 THIN SPACE h s
0x200a HAIR SPACE h s
0x2028 LINE SEPARATOR vs
0x2029 PARAGRAPH SEPARATOR vs
0x202f NARROW NO-BREAK SPACE h s
0x205f MEDIUM MATHEMATICAL SPACE h s
0x3000 IDEOGRAPHIC SPACE h s
- [1]
- Prior to Perl v5.18, "\s" did not match the vertical tab. The
change in v5.18 is considered an experiment, which means it could be
backed out in v5.22 if experience indicates that it breaks too much
existing code. If this change adversely affects you, send email to
"perlbug@perl.org"; if it affects you positively, email
"perlthanks@perl.org". In the meantime, "[^\S\cK]"
(obscurely) matches what "\s" traditionally did.
- [2]
- NEXT LINE and NO-BREAK SPACE may or may not match "\s" depending
on the rules in effect. See the beginning of this section.
Unicode Properties
"\pP" and "\p{Prop}" are character classes to match
characters that fit given Unicode properties. One letter property names can be
used in the "\pP" form, with the property name following the
"\p", otherwise, braces are required. When using braces, there is a
single form, which is just the property name enclosed in the braces, and a
compound form which looks like "\p{name=value}", which means to
match if the property "name" for the character has that particular
"value". For instance, a match for a number can be written as
"/\pN/" or as "/\p{Number}/", or as
"/\p{Number=True}/". Lowercase letters are matched by the property
Lowercase_Letter which has the short form
Ll. They need the
braces, so are written as "/\p{Ll}/" or
"/\p{Lowercase_Letter}/", or
"/\p{General_Category=Lowercase_Letter}/" (the underscores are
optional). "/\pLl/" is valid, but means something different. It
matches a two character string: a letter (Unicode property "\pL"),
followed by a lowercase "l".
If locale rules are not in effect, the use of a Unicode property will force the
regular expression into using Unicode rules, if it isn't already.
Note that almost all properties are immune to case-insensitive matching. That
is, adding a "/i" regular expression modifier does not change what
they match. There are two sets that are affected. The first set is
"Uppercase_Letter", "Lowercase_Letter", and
"Titlecase_Letter", all of which match "Cased_Letter"
under "/i" matching. The second set is "Uppercase",
"Lowercase", and "Titlecase", all of which match
"Cased" under "/i" matching. (The difference between these
sets is that some things, such as Roman numerals, come in both upper and lower
case, so they are "Cased", but aren't considered to be letters, so
they aren't "Cased_Letter"s. They're actually
"Letter_Number"s.) This set also includes its subsets
"PosixUpper" and "PosixLower", both of which under
"/i" match "PosixAlpha".
For more details on Unicode properties, see "Unicode Character
Properties" in perlunicode; for a complete list of possible properties,
see "Properties accessible through \p{} and \P{}" in perluniprops,
which notes all forms that have "/i" differences. It is also
possible to define your own properties. This is discussed in
"User-Defined Character Properties" in perlunicode.
Unicode properties are defined (surprise!) only on Unicode code points. Starting
in v5.20, when matching against "\p" and "\P", Perl treats
non-Unicode code points (those above the legal Unicode maximum of 0x10FFFF) as
if they were typical unassigned Unicode code points.
Prior to v5.20, Perl raised a warning and made all matches fail on non-Unicode
code points. This could be somewhat surprising:
chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails on Perls < v5.20.
chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Also fails on Perls
# < v5.20
Even though these two matches might be thought of as complements, until v5.20
they were so only on Unicode code points.
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 whitespace.
"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-whitespace.
" " =~ /\h/ # Match, space is horizontal whitespace.
" " =~ /\v/ # No match, space is not vertical whitespace.
"\r" =~ /\v/ # Match, a return is vertical whitespace.
"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 Laotian character.
It is worth emphasizing that "\d", "\w", etc, match single
characters, not complete numbers or words. To match a number (that consists of
digits), use "\d+"; to match a word, use "\w+". But be
aware of the security considerations in doing so, as mentioned above.
Bracketed Character Classes¶
The third form of character class you can use in Perl regular expressions is the
bracketed character class. In its simplest form, it lists the characters that
may be matched, surrounded by square brackets, like this: "[aeiou]".
This matches one of "a", "e", "i", "o"
or "u". Like the other character classes, exactly one character is
matched.* To match a longer string consisting of characters mentioned in the
character class, follow the character class with a quantifier. For instance,
"[aeiou]+" matches one or more lowercase English 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.
-------
* There is an exception to a bracketed character class matching a single
character only. When the class is to match caselessly under "/i"
matching rules, and a character that is explicitly mentioned inside the class
matches a multiple-character sequence caselessly under Unicode rules, the
class (when not inverted) will also match that sequence. For example, Unicode
says that the letter "LATIN SMALL LETTER SHARP S" should match the
sequence "ss" under "/i" rules. Thus,
'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i # Matches
'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i # Matches
For this to happen, the character must be explicitly specified, and not be part
of a multi-character range (not even as one of its endpoints).
("Character Ranges" will be explained shortly.) Therefore,
'ss' =~ /\A[\0-\x{ff}]\z/i # Doesn't match
'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/i # No match
'ss' =~ /\A[\xDF-\xDF]\z/i # Matches on ASCII platforms, since \XDF
# is LATIN SMALL LETTER SHARP S, and the
# range is just a single element
Note that it isn't a good idea to specify these types of ranges anyway.
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.
The sequences "\a", "\c", "\e", "\f",
"\n", "\N{
NAME}", "\N{U+
hex
char}", "\r", "\t", and "\x" are also
special and have the same meanings as they do outside a bracketed character
class. (However, inside a bracketed character class, if "\N{
NAME}" expands to a sequence of characters, only the first one in
the sequence is used, with a warning.)
Also, a backslash followed by two or three octal digits is considered an octal
number.
A "[" is not special inside a character class, unless it's the start
of a POSIX character class (see "POSIX Character Classes" below). It
normally does not need escaping.
A "]" is normally either the end of a POSIX character class (see
"POSIX Character Classes" below), or it signals the end of the
bracketed character class. If you want to include a "]" in the set
of characters, you must generally escape it.
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 to 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 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 characters between the two were 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 necessarily
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 means. 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 syntactically 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 so is
considered a character to be matched literally. If you want a hyphen in your
set of characters to be matched and its position in the class is such that it
could be considered part of a range, you must escape that hyphen with a
backslash.
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 any character that
is not a lowercase ASCII letter, which therefore includes more than a million
Unicode code points. The class is said to be "negated" or
"inverted".
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 the
caret as one of the characters to match, either escape the caret or else don't
list it first.
In inverted bracketed character classes, Perl ignores the Unicode rules that
normally say that certain characters should match a sequence of multiple
characters under caseless "/i" matching. Following those rules could
lead to highly confusing situations:
"ss" =~ /^[^\xDF]+$/ui; # Matches!
This should match any sequences of characters that aren't "\xDF" nor
what "\xDF" matches under "/i". "s" isn't
"\xDF", but Unicode says that "ss" is what
"\xDF" matches under "/i". So which one "wins"?
Do you fail the match because the string has "ss" or accept it
because it has an "s" followed by another "s"? Perl has
chosen the latter.
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 any backslash sequence character class (with the exception of
"\N" and "\R") inside a bracketed character class, and it
will act just as if you had put all characters matched by the backslash
sequence inside the character class. For instance, "[a-f\d]" matches
any decimal digit, or any of the lowercase letters between 'a' and 'f'
inclusive.
"\N" within a bracketed character class must be of the forms "\N{
name}" or "\N{U+
hex char}", and NOT be the form
that matches non-newlines, for the same reason that a dot "." inside
a bracketed character class loses its special meaning: it matches nearly
anything, which generally isn't what you want to happen.
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. Thus, you can't say:
/[\p{Thai}-\d]/ # Wrong!
POSIX Character Classes
POSIX character classes have the form "[:class:]", where
class
is the name, and the "[:" and ":]" delimiters. POSIX
character classes only 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".
POSIX character classes can be part of a larger bracketed character class. For
example,
[01[:alpha:]%]
is valid and matches '0', '1', any alphabetic character, and the percent sign.
Perl recognizes the following POSIX character classes:
alpha Any alphabetical character ("[A-Za-z]").
alnum Any alphanumeric character ("[A-Za-z0-9]").
ascii Any character in the ASCII character set.
blank A GNU extension, equal to a space or a horizontal tab ("\t").
cntrl Any control character. See Note [2] below.
digit Any decimal digit ("[0-9]"), equivalent to "\d".
graph Any printable character, excluding a space. See Note [3] below.
lower Any lowercase character ("[a-z]").
print Any printable character, including a space. See Note [4] below.
punct Any graphical character excluding "word" characters. Note [5].
space Any whitespace character. "\s" including the vertical tab
("\cK").
upper Any uppercase character ("[A-Z]").
word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
xdigit Any hexadecimal digit ("[0-9a-fA-F]").
Most POSIX character classes have two Unicode-style "\p" property
counterparts. (They are not official Unicode properties, but Perl extensions
derived from official Unicode properties.) The table below shows the relation
between POSIX character classes and these counterparts.
One counterpart, in the column labelled "ASCII-range Unicode" in the
table, matches only characters in the ASCII character set.
The other counterpart, in the column labelled "Full-range Unicode",
matches any appropriate characters in the full Unicode character set. For
example, "\p{Alpha}" matches not just the ASCII alphabetic
characters, but any character in the entire Unicode character set considered
alphabetic. An entry in the column labelled "backslash sequence" is
a (short) equivalent.
[[:...:]] ASCII-range Full-range backslash Note
Unicode Unicode sequence
-----------------------------------------------------
alpha \p{PosixAlpha} \p{XPosixAlpha}
alnum \p{PosixAlnum} \p{XPosixAlnum}
ascii \p{ASCII}
blank \p{PosixBlank} \p{XPosixBlank} \h [1]
or \p{HorizSpace} [1]
cntrl \p{PosixCntrl} \p{XPosixCntrl} [2]
digit \p{PosixDigit} \p{XPosixDigit} \d
graph \p{PosixGraph} \p{XPosixGraph} [3]
lower \p{PosixLower} \p{XPosixLower}
print \p{PosixPrint} \p{XPosixPrint} [4]
punct \p{PosixPunct} \p{XPosixPunct} [5]
\p{PerlSpace} \p{XPerlSpace} \s [6]
space \p{PosixSpace} \p{XPosixSpace} [6]
upper \p{PosixUpper} \p{XPosixUpper}
word \p{PosixWord} \p{XPosixWord} \w
xdigit \p{PosixXDigit} \p{XPosixXDigit}
- [1]
- "\p{Blank}" and "\p{HorizSpace}" are synonyms.
- [2]
- Control characters don't produce output as such, but instead usually
control the terminal somehow: for example, newline and backspace are
control characters. In the ASCII range, characters whose code points are
between 0 and 31 inclusive, plus 127 ("DEL") are control
characters.
- [3]
- Any character that is graphical, that is, visible. This class
consists of all alphanumeric characters and all punctuation
characters.
- [4]
- All printable characters, which is the set of all graphical characters
plus those whitespace characters which are not also controls.
- [5]
- "\p{PosixPunct}" and "[[:punct:]]" in the ASCII range
match all non-controls, non-alphanumeric, non-space characters:
"[-!"#$%&'()*+,./:;<=>?@[\\\]^_`{|}~]" (although
if a locale is in effect, it could alter the behavior of
"[[:punct:]]").
The similarly named property, "\p{Punct}", matches a somewhat
different set in the ASCII range, namely
"[-!"#%&'()*,./:;?@[\\\]_{}]". That is, it is missing
the nine characters "[$+<=>^`|~]". This is because Unicode
splits what POSIX considers to be punctuation into two categories,
Punctuation and Symbols.
"\p{XPosixPunct}" and (under Unicode rules)
"[[:punct:]]", match what "\p{PosixPunct}" matches in
the ASCII range, plus what "\p{Punct}" matches. This is
different than strictly matching according to "\p{Punct}".
Another way to say it is that if Unicode rules are in effect,
"[[:punct:]]" matches all characters that Unicode considers
punctuation, plus all ASCII-range characters that Unicode considers
symbols.
- [6]
- "\p{XPerlSpace}" and "\p{Space}" match identically
starting with Perl v5.18. In earlier versions, these differ only in that
in non-locale matching, "\p{XPerlSpace}" does not match the
vertical tab, "\cK". Same for the two ASCII-only range
forms.
There are various other synonyms that can be used besides the names listed in
the table. For example, "\p{PosixAlpha}" can be written as
"\p{Alpha}". All are listed in "Properties accessible through
\p{} and \P{}" in perluniprops.
Both the "\p" counterparts always assume Unicode rules are in effect.
On ASCII platforms, this means they assume that the code points from 128 to
255 are Latin-1, and that means that using them under locale rules is unwise
unless the locale is guaranteed to be Latin-1 or UTF-8. In contrast, the POSIX
character classes are useful under locale rules. They are affected by the
actual rules in effect, as follows:
- If the "/a" modifier, is in effect ...
- Each of the POSIX classes matches exactly the same as their ASCII-range
counterparts.
- otherwise ...
- For code points above 255 ...
- The POSIX class matches the same as its Full-range counterpart.
- For code points below 256 ...
- if locale rules are in effect ...
- The POSIX class matches according to the locale, except:
- "word"
- also includes the platform's native underscore character, no matter what
the locale is.
- "ascii"
- on platforms that don't have the POSIX "ascii" extension, this
matches just the platform's native ASCII-range characters.
- "blank"
- on platforms that don't have the POSIX "blank" extension, this
matches just the platform's native tab and space characters.
- if Unicode rules are in effect ...
- The POSIX class matches the same as the Full-range counterpart.
- otherwise ...
- The POSIX class matches the same as the ASCII range counterpart.
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
It is proposed to change this behavior in a future release of Perl so that
whether or not Unicode rules are in effect would not change the behavior:
Outside of locale, the POSIX classes would behave like their ASCII-range
counterparts. If you wish to comment on this proposal, send email to
"perl5-porters@perl.org".
Negation of POSIX character classes
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 ASCII-range Full-range backslash
Unicode Unicode sequence
-----------------------------------------------------
[[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D
[[:^space:]] \P{PosixSpace} \P{XPosixSpace}
\P{PerlSpace} \P{XPerlSpace} \S
[[:^word:]] \P{PerlWord} \P{XPosixWord} \W
The backslash sequence can mean either ASCII- or Full-range Unicode, depending
on various factors as described in "Which character set modifier is in
effect?" in perlre.
[= =] and [. .]
Perl recognizes the POSIX character classes "[=class=]" and
"[.class.]", but does not (yet?) support them. Any attempt to use
either construct raises an exception.
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
# except the letters 'a' to 'f' and 'A' to
# 'F'. This is because the main character
# class is composed of two POSIX character
# classes that are ORed together, one that
# matches any digit, and the other that
# matches anything that isn't a hex digit.
# The OR adds the digits, leaving only the
# letters 'a' to 'f' and 'A' to 'F' excluded.
Extended Bracketed Character Classes
This is a fancy bracketed character class that can be used for more readable and
less error-prone classes, and to perform set operations, such as intersection.
An example is
/(?[ \p{Thai} & \p{Digit} ])/
This will match all the digit characters that are in the Thai script.
This is an experimental feature available starting in 5.18, and is subject to
change as we gain field experience with it. Any attempt to use it will raise a
warning, unless disabled via
no warnings "experimental::regex_sets";
Comments on this feature are welcome; send email to
"perl5-porters@perl.org".
We can extend the example above:
/(?[ ( \p{Thai} + \p{Lao} ) & \p{Digit} ])/
This matches digits that are in either the Thai or Laotian scripts.
Notice the white space in these examples. This construct always has the
"/x" modifier turned on within it.
The available binary operators are:
& intersection
+ union
| another name for '+', hence means union
- subtraction (the result matches the set consisting of those
code points matched by the first operand, excluding any that
are also matched by the second operand)
^ symmetric difference (the union minus the intersection). This
is like an exclusive or, in that the result is the set of code
points that are matched by either, but not both, of the
operands.
There is one unary operator:
! complement
All the binary operators left associate, and are of equal precedence. The unary
operator right associates, and has higher precedence. Use parentheses to
override the default associations. Some feedback we've received indicates a
desire for intersection to have higher precedence than union. This is
something that feedback from the field may cause us to change in future
releases; you may want to parenthesize copiously to avoid such changes
affecting your code, until this feature is no longer considered experimental.
The main restriction is that everything is a metacharacter. Thus, you cannot
refer to single characters by doing something like this:
/(?[ a + b ])/ # Syntax error!
The easiest way to specify an individual typable character is to enclose it in
brackets:
/(?[ [a] + [b] ])/
(This is the same thing as "[ab]".) You could also have said the
equivalent:
/(?[[ a b ]])/
(You can, of course, specify single characters by using, "\x{...}",
"\N{...}", etc.)
This last example shows the use of this construct to specify an ordinary
bracketed character class without additional set operations. Note the white
space within it; "/x" is turned on even within bracketed character
classes, except you can't have comments inside them. Hence,
(?[ [#] ])
matches the literal character "#". To specify a literal white space
character, you can escape it with a backslash, like:
/(?[ [ a e i o u \ ] ])/
This matches the English vowels plus the SPACE character. All the other escapes
accepted by normal bracketed character classes are accepted here as well; but
unrecognized escapes that generate warnings in normal classes are fatal errors
here.
All warnings from these class elements are fatal, as well as some practices that
don't currently warn. For example you cannot say
/(?[ [ \xF ] ])/ # Syntax error!
You have to have two hex digits after a braceless "\x" (use a leading
zero to make two). These restrictions are to lower the incidence of typos
causing the class to not match what you thought it would.
If a regular bracketed character class contains a "\p{}" or
"\P{}" and is matched against a non-Unicode code point, a warning
may be raised, as the result is not Unicode-defined. No such warning will come
when using this extended form.
The final difference between regular bracketed character classes and these, is
that it is not possible to get these to match a multi-character fold. Thus,
/(?[ [\xDF] ])/iu
does not match the string "ss".
You don't have to enclose POSIX class names inside double brackets, hence both
of the following work:
/(?[ [:word:] - [:lower:] ])/
/(?[ [[:word:]] - [[:lower:]] ])/
Any contained POSIX character classes, including things like "\w" and
"\D" respect the "/a" (and "/aa") modifiers.
"(?[ ])" is a regex-compile-time construct. Any attempt to use
something which isn't knowable at the time the containing regular expression
is compiled is a fatal error. In practice, this means just three limitations:
- 1.
- This construct cannot be used within the scope of "use locale"
(or the "/l" regex modifier).
- 2.
- Any user-defined property used must be already defined by the time the
regular expression is compiled (but note that this construct can be used
instead of such properties).
- 3.
- A regular expression that otherwise would compile using "/d"
rules, and which uses this construct will instead use "/u". Thus
this construct tells Perl that you don't want "/d" rules for the
entire regular expression containing it.
The "/x" processing within this class is an extended form. Besides the
characters that are considered white space in normal "/x"
processing, there are 5 others, recommended by the Unicode standard:
U+0085 NEXT LINE
U+200E LEFT-TO-RIGHT MARK
U+200F RIGHT-TO-LEFT MARK
U+2028 LINE SEPARATOR
U+2029 PARAGRAPH SEPARATOR
Note that skipping white space applies only to the interior of this construct.
There must not be any space between any of the characters that form the
initial "(?[". Nor may there be space between the closing
"])" characters.
Just as in all regular expressions, the pattern can be built up by including
variables that are interpolated at regex compilation time. Care must be taken
to ensure that you are getting what you expect. For example:
my $thai_or_lao = '\p{Thai} + \p{Lao}';
...
qr/(?[ \p{Digit} & $thai_or_lao ])/;
compiles to
qr/(?[ \p{Digit} & \p{Thai} + \p{Lao} ])/;
But this does not have the effect that someone reading the code would likely
expect, as the intersection applies just to "\p{Thai}", excluding
the Laotian. Pitfalls like this can be avoided by parenthesizing the component
pieces:
my $thai_or_lao = '( \p{Thai} + \p{Lao} )';
But any modifiers will still apply to all the components:
my $lower = '\p{Lower} + \p{Digit}';
qr/(?[ \p{Greek} & $lower ])/i;
matches upper case things. You can avoid surprises by making the components into
instances of this construct by compiling them:
my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
my $lower = qr/(?[ \p{Lower} + \p{Digit} ])/;
When these are embedded in another pattern, what they match does not change,
regardless of parenthesization or what modifiers are in effect in that outer
pattern.
Due to the way that Perl parses things, your parentheses and brackets may need
to be balanced, even including comments. If you run into any examples, please
send them to "perlbug@perl.org", so that we can have a concrete
example for this man page.
We may change it so that things that remain legal uses in normal bracketed
character classes might become illegal within this experimental construct. One
proposal, for example, is to forbid adjacent uses of the same character, as in
"(?[ [aa] ])". The motivation for such a change is that this usage
is likely a typo, as the second "a" adds nothing.