Regular Expression Wiki

In the BRE syntax, most characters are treated as literals — they match only themselves (e.g., a matches "a"). The exceptions, listed below, are called metacharacters or metasequences.

Metacharacter	Description
.	Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
[ ]	A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first (after the ^) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].
[^ ]	Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.
^	Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
$	Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
BRE: \( \) ERE: ( )	Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group.
\n	Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is theoretically irregular and was not adopted in the POSIX ERE syntax. Some tools allow referencing more than nine capturing groups.
*	Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. \(ab\)* matches "", "ab", "abab", "ababab", and so on.
BRE: \{m,n\} ERE: {m,n}	Matches the preceding element at least m and not more than n times. For example, a\{3,5\} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regular expressions.

Examples:

• .at matches any three-character string ending with "at", including "hat", "cat", and "bat".
• [hc]at matches "hat" and "cat".
• [^b]at matches all strings matched by .at except "bat".
• ^[hc]at matches "hat" and "cat", but only at the beginning of the string or line.
• [hc]at$ matches "hat" and "cat", but only at the end of the string or line.
• \[.\] matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".

POSIX Extended Regular Expressions

The meaning of metacharacters escaped with a backslash is reversed for some characters in the POSIX Extended Regular Expression (ERE) syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. So, for example, \( \) is now ( ) and \{ \} is now { }. Additionally, support is removed for \n backreferences and the following metacharacters are added:

Metacharacter	Description
?	Matches the preceding element zero or one time. For example, ba? matches "b" or "ba".
+	Matches the preceding element one or more times. For example, ba+ matches "ba", "baa", "baaa", and so on.
|	The choice (aka alternation or set union) operator matches either the expression before or the expression after the operator. For example, abc|def matches "abc" or "def".

Examples:

• [hc]+at matches "hat", "cat", "hhat", "chat", "hcat", "ccchat", and so on, but not "at".
• [hc]?at matches "hat", "cat", and "at".
• [hc]*at matches "hat", "cat", "hhat", "chat", "hcat", "ccchat", "at", and so on.
• cat|dog matches "cat" or "dog".

POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the command line flag -E.

POSIX character classes

Since many ranges of characters depend on the chosen locale setting (i.e., in some settings letters are organized as abc...zABC...Z, while in some others as aAbBcC...zZ), the POSIX standard defines some classes or categories of characters as shown in the following table:

POSIX	Non-standard	Perl	Vim	ASCII	Description
[:alnum:]				[A-Za-z0-9]	Alphanumeric characters
	[:word:]	\w	\w	[A-Za-z0-9_]	Alphanumeric characters plus "_"
		\W	\W	[^A-Za-z0-9_]	Non-word characters
[:alpha:]			\a	[A-Za-z]	Alphabetic characters
[:blank:]				[ \t]	Space and tab
		\b	\< \>	(?<=\W)(?=\w)|(?<=\w)(?=\W)	Word boundaries
[:cntrl:]				[\x00-\x1F\x7F]	Control characters
[:digit:]		\d	\d	[0-9]	Digits
		\D	\D	[^0-9]	Non-digits
[:graph:]				[\x21-\x7E]	Visible characters
[:lower:]			\l	[a-z]	Lowercase letters
[:print:]			\p	[\x20-\x7E]	Visible characters and the space character
[:punct:]				[\]\[!"#$%&'()*+,./:;<=>?@\^_`{|}~-]	Punctuation characters
[:space:]		\s	\s	[ \t\r\n\v\f]	Whitespace characters
		\S	\S	[^ \t\r\n\v\f]	Non-whitespace characters
[:upper:]			\u	[A-Z]	Uppercase letters
[:xdigit:]			\x	[A-Fa-f0-9]	Hexadecimal digits

Metacharacter(s)	Description	Example Note that all the if statements return a TRUE value
.	Normally matches any character except a newline. Within square brackets the dot is literal.	$string1 = "Hello World\n"; if ($string1 =~ m/...../) { print "$string1 has length >= 5\n"; }
( )	Groups a series of pattern elements to a single element. When you match a pattern within parentheses, you can use any of $1, $2, ... later to refer to the previously matched pattern.	$string1 = "Hello World\n"; if ($string1 =~ m/(H..).(o..)/) { print "We matched '$1' and '$2'\n"; } Output: We matched 'Hel' and 'o W';
+	Matches the preceding pattern element one or more times.	$string1 = "Hello World\n"; if ($string1 =~ m/l+/) { print "There are one or more consecutive letter \"l\"'s in $string1\n"; } Output: There are one or more consecutive letter "l"'s in Hello World
?	Matches the preceding pattern element zero or one times.	$string1 = "Hello World\n"; if ($string1 =~ m/H.?e/) { print "There is an 'H' and a 'e' separated by "; print "0-1 characters (Ex: He Hoe)\n"; }
?	Modifies the *, +, or {M,N}'d regex that comes before to match as few times as possible.	$string1 = "Hello World\n"; if ($string1 =~ m/(l.+?o)/) { print "The non-greedy match with 'l' followed by one or "; print "more characters is 'llo' rather than 'llo wo'.\n"; }
*	Matches the preceding pattern element zero or more times.	$string1 = "Hello World\n"; if ($string1 =~ m/el*o/) { print "There is an 'e' followed by zero to many "; print "'l' followed by 'o' (eo, elo, ello, elllo)\n"; }
{M,N}	Denotes the minimum M and the maximum N match count.	$string1 = "Hello World\n"; if ($string1 =~ m/l{1,2}/) { print "There exists a substring with at least 1 "; print "and at most 2 l's in $string1\n"; }
[...]	Denotes a set of possible character matches.	$string1 = "Hello World\n"; if ($string1 =~ m/[aeiou]+/) { print "$string1 contains one or more vowels.\n"; }
|	Separates alternate possibilities.	$string1 = "Hello World\n"; if ($string1 =~ m/(Hello|Hi|Pogo)/) { print "At least one of Hello, Hi, or Pogo is "; print "contained in $string1.\n"; }
\b	Matches a zero-width boundary between a word-class character (see next) and either a non-word class character or an edge.	$string1 = "Hello World\n"; if ($string1 =~ m/llo\b/) { print "There is a word that ends with 'llo'\n"; }
\w	Matches an alphanumeric character, including "_"; same as [A-Za-z0-9_] in ASCII. In Unicode[14] same as [\p{Alphabetic}\p{GC=Mark}\p{GC=Decimal_Number\p{GC=Connector_Punctuation}], where the Alphabetic property contains more than just Letters, and the Decimal_Number property contains more than [0-9].	$string1 = "Hello World\n"; if ($string1 =~ m/\w/) { print "There is at least one alphanumeric "; print "character in $string1 (A-Z, a-z, 0-9, _)\n"; }
\W	Matches a non-alphanumeric character, excluding "_"; same as [^A-Za-z0-9_] in ASCII, and [^\p{Alphabetic}\p{GC=Mark}\p{GC=Decimal_Number}\p{GC=Connector_Punctuation}] in Unicode.	$string1 = "Hello World\n"; if ($string1 =~ m/\W/) { print "The space between Hello and "; print "World is not alphanumeric\n"; }
\s	Matches a whitespace character, which in ASCII are tab, line feed, form feed, carriage return, and space; in Unicode, also matches no-break spaces, next line, and the variable-width spaces (amongst others).	$string1 = "Hello World\n"; if ($string1 =~ m/\s.*\s/) { print "There are TWO whitespace characters, which may"; print " be separated by other characters, in $string1"; }
\S	Matches anything BUT a whitespace.	$string1 = "Hello World\n"; if ($string1 =~ m/\S.*\S/) { print "There are TWO non-whitespace characters, which"; print " may be separated by other characters, in $string1"; }
\d	Matches a digit; same as [0-9] in ASCII; in Unicode, same as the \p{Digit} or \p{GC=Decimal_Number} property, which itself the same as the \p{Numeric_Type=Decimal} property.	$string1 = "99 bottles of beer on the wall."; if ($string1 =~ m/(\d+)/) { print "$1 is the first number in '$string1'\n"; } Output: 99 is the first number in '99 bottles of beer on the wall.'
\D	Matches a non-digit; same as [^0-9] in ASCII or \P{Digit} in Unicode.	$string1 = "Hello World\n"; if ($string1 =~ m/\D/) { print "There is at least one character in $string1"; print " that is not a digit.\n"; }
^	Matches the beginning of a line or string.	$string1 = "Hello World\n"; if ($string1 =~ m/^He/) { print "$string1 starts with the characters 'He'\n"; }
$	Matches the end of a line or string.	$string1 = "Hello World\n"; if ($string1 =~ m/rld$/) { print "$string1 is a line or string "; print "that ends with 'rld'\n"; }
\A	Matches the beginning of a string (but not an internal line).	$string1 = "Hello\nWorld\n"; if ($string1 =~ m/\AH/) { print "$string1 is a string "; print "that starts with 'H'\n"; }
\z	Matches the end of a string (but not an internal line). see Perl Best Practices — Page 240	$string1 = "Hello\nWorld\n"; if ($string1 =~ m/d\n\z/) { print "$string1 is a string "; print "that ends with 'd\\n'\n"; }
[^...]	Matches every character except the ones inside brackets.	$string1 = "Hello World\n"; if ($string1 =~ m/[^abc]/) { print "$string1 contains a character other than "; print "a, b, and c\n"; }