sed regular expression

^	matches the beginning of the line
$	matches the end of the line
.	Matches any single character
(character)*	match arbitrarily many occurences of (character)
(character)?	Match 0 or 1 instance of (character)
[abcdef]	Match any character enclosed in [] (in this instance, a b c d e or f) ranges of characters such as [a-z] are permitted. The behaviour of this deserves more description. See the page on grep for more details about the syntax of lists.
[^abcdef]	Match any character NOT enclosed in [] (in this instance, any character other than a b c d e or f)
(character)\{m,n\}	Match m-n repetitions of (character)
(character)\{m,\}	Match m or more repetitions of (character)
(character)\{,n\}	Match n or less (possibly 0) repetitions of (character)
(character)\{n\}	Match exactly n repetitions of (character)
\(expression\)	Group operator.
\n	Backreference - matches nth group
expression1\|expression2	Matches expression1 or expression 2. Works with GNU sed, but this feature might not work with other forms of sed.

A very useful link about sed is:

 http://www.ibm.com/developerworks/linux/library/l-sed2/index.html