Regex

Check out rubular.

[abc]		A single character of: a, b, or c
[^abc]		Any single character except: a, b, or c
[a-z]		Any single character in the range a-z
[a-zA-Z]	Any single character in the range a-z or A-Z
^		Start of line
$		End of line
\A		Start of string
\z		End of string
.		Any single character
\s		Any whitespace character
\S		Any non-whitespace character
\d		Any digit
\D		Any non-digit
\w		Any word character (letter, number, underscore)
\W		Any non-word character
\b		Any word boundary
(...)		Capture everything enclosed
(a|b)		a or b
a?		Zero or one of a
a*		Zero or more of a
a+		One or more of a
a{3}		Exactly 3 of a
a{3,}		3 or more of a
a{3,6}		Between 3 and 6 of a

options:

i case insensitive 
m make dot match newlines 
x ignore whitespace in regex 
o perform #{...} substitutions only once