Pattern Matching Tips

Grep Regular Expression
^	Denotes the beginning of a line
$	Denotes the end of a line
.	Matches any one characters
*	Matches 0 or more of the previous characters
.*	Matches any number or type of characters
[]	Matches on character for the one listed in the the Square brackets
[^]	Does not match any characters listed
<, >/	Denotes the beginning and end (respectively) of a word

So an example of a regular expression search would be

% grep "<[A-Za-z].*" file

This will search for any word which begins with a letter upper or lower case. 

Sed Replace Expression

To remove the comments from an apt sources.list file use:

sudo sed -i -e "s/# deb/deb/g" /etc/apt/sources.list

Regular Expression Operators

a?	matches 0 or 1 occurrence of *a*	'a' or empty string
a*	matches 0 or more occurrences of *a*	empty string or 'a', 'aa', 'aaa', etc
a+	matches 1 or more occurrences of *a*	'a', 'aa', 'aaa', etc
a|b	match *a* or *b*	'a' or 'b' -
.	match any single character	'a', 'q', 'l', '_', '+', etc
[woeirjsd]	match any of the named characters	'w', 'o', 'e', 'i', 'r', 'j', 's', 'd'
[1-9]	match any of the characters in the range	'1', '2', '3', '4', '5', '6', '7', '8', '9'
[^13579]	match any characters not named	even digits, or any other character
(ie)	group an expression (for use with other operators)	'ie'
^a	match an *a* at the beginning of a line	'a'
a$	match an *a* at the end of a line	'a'

Example:

/	[^?]	+	?	/
begin expression	any character other than '?'	more than one of those	a question mark	end expression

So the use of the in front of the ? makes it refer to an actual question mark.