Regular Expression

It's a very elegant summary of regular expression from The AWK Programming Language.

1. The regular expression metacharacters are:

\ ^ $ . [ ] | ( ) * + ?

2. A basic regular expression is one of the following:

• a nonmetacharacter, such as A, that matches itself.
• an escape sequence that matches a special symbol: \t matches a tab.
• a quoted metacharacter, such as \*, that matches the metaqcharacter literally.
• ^, which matches the beginning of a string.
• $, which matches the end of a string.
• ., which matches any single character.
• a character class: [ABC] matches any of the characters A, B, or C. Character classes may include abbreviations: [A-Za-z] matches any single letter.
• a complemented character class: [^0-9] matches any character except a digit.

3. These operators combine regular expressions into larger ones:

• alternation: A | B matches A or B.
• concatenation: AB matches A immediately followed by B.
• closure: A* matches zero or more A's.
• positive closure: A+ matches one or more A's.
• zero or one: A? matches the null string or A.
• parentheses: (r) matches the same strings as r does.

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Regular Expressions

Expression	Matches
c	the nonmetacharacter c
\c	escape sequence or literal character c
^	beginning of string
$	end of string
.	any character
[$c_1$$c_2$...]	any character in $c_1$$c_2$
[^$c_1$$c_2$...]	any character not in $c_1$$c_2$
[$c_1$-$c_2$]	any character in the range beginning with $c_1$ and ending with $c_2$
[^$c_1$-$c_2$]	any character not in the range $c_1$ to $c_2$
$r_1$|$r_2$	any string matched by $r_1$ or $r_2$
($r_1$)($r_2$)	any string xy where $r_1$ matches x and $r_2$ matches y; parentheses not needed around arguments with no alternations
(r)*	zero or more consecutive strings matched by r
(r)+	one or more consecutive strings matched by r
(r)?	zero or one string matched by r parentheses not needed around basic regular expressions
(r)	any string matched by r