Special Characters

You can use special characters within a regular expression to cause the regular expression to match more than one string. A regular expression that includes a special character always matches the longest possible string, starting as far toward the beginning (left) of the line as possible.

Periods

A period (.) matches any character (Table A-2).

Table A-2. Period

Regular expression	Matches	Examples
/ .alk/	All strings consisting of a SPACE followed by any character followed by alk	will talk, may balk
/.ing/	All strings consisting of any character preceding ing	sing song, ping, before inglenook

Brackets

Brackets ( [ ] ) define a character class^[1] that matches any single character within the brackets (Table A-3). If the first character following the left bracket is a caret (^), the brackets define a character class that matches any single character not within the brackets. You can use a hyphen to indicate a range of characters. Within a character-class definition, backslashes and asterisks (described in the following sections) lose their special meanings. A right bracket (appearing as a member of the character class) can appear only as the first character following the left bracket. A caret is special only if it is the first character following the left bracket. A dollar sign is special only if it is followed immediately by the right bracket.

^[1] GNU documentation calls these List Operators and defines Character Class operators as expressions that match a predefined group of characters, such as all numbers (see Table V-28 on page 804).

Table A-3. Brackets

Regular expression	Matches	Examples
/[bB]ill/	Member of the character class b and B followed by ill	bill, Bill, billed
/t[aeiou].k/	t followed by a lowercase vowel, any character, and a k	talkative, stink, teak, tanker
/# [6 9]/	# followed by a SPACE and a member of the character class 6 through 9	# 60, # 8:, get # 9
/[^a zA Z]/	Any character that is not a letter (ASCII character set only)	1, 7, @, ., }, Stop!

Asterisks

An asterisk can follow a regular expression that represents a single character (Table A-4). The asterisk represents zero or more occurrences of a match of the regular expression. An asterisk following a period matches any string of characters. (A period matches any character, and an asterisk matches zero or more occurrences of the preceding regular expression.) A character-class definition followed by an asterisk matches any string of characters that are members of the character class.

Table A-4. Asterisks

Regular expression	Matches	Examples
/ab*c/	a followed by zero or more b's followed by a c	ac, abc, abbc, debbcaabbbc
/ab.*c/	ab followed by zero or more characters followed by c	abc, abxc, ab45c, xab 756.345 x cat
/t.*ing/	t followed by zero or more characters followed by ing	thing, ting, I thought of going
/[a zA Z ]*/	A string composed only of letters and SPACEs	1. any string without numbers or punctuation!
/(.*)/	As long a string as possible between ( and )	Get (this) and (that);
/([^)]*)/	The shortest string possible that starts with ( and ends with )	(this), Get (this and that)

Carets and Dollar Signs

A regular expression that begins with a caret (^) can match a string only at the beginning of a line. In a similar manner, a dollar sign ($) at the end of a regular expression matches the end of a line. The caret and dollar sign are called anchors because they force (anchor) a match to the beginning or end of a line (Table A-5).

Table A-5. Carets and dollar signs

Regular expression	Matches	Examples
/^T/	A T at the beginning of a line	This line..., That Time..., In Time
/^+[0 9]/	A plus sign followed by a digit at the beginning of a line	+5 +45.72, +759 Keep this...
/:$/	A colon that ends a line	...below:

Quoting Special Characters

You can quote any special character (but not a digit or a parenthesis) by preceding it with a backslash (Table A-6). Quoting a special character makes it represent itself.

Table A-6. Quoted special characters

Regular expression	Matches	Examples
/end \ ./	All strings that contain end followed by a period	The end., send., pretend.mail
/ \\ /	A single backslash	\
/ \*/	An asterisk	*.c, an asterisk (*)
/ \[5 \] /	[5]	it was five [5]
/and\ /or /	and/or	and/or