• Red Hat Linux 6 Unleashed Chapter 34 (link here, but not sure how legal that page is, or how long it will remain there)
• Regular Expression HowTo
• An Introduction to Python

Character Escape Codes

 \               Newline Continuation
 \\              Backslash
 \'              Single quote
 \"              Double quote
 \a              Bell
 \b              Backspace
 \e              Escape
 \n              Line feed
 \v              Vertical tab
 \t              Horizontal tab
 \r              Carriage return
 \0              Null
 \0XX            Octal character value
 \xXX            Hex character value

Regular Expression Metacharacters

.    		Any character
^    		Beginning of the string
$    		End of the string
\s    		Whitespace character
\S    		Non-whitespace character
\d    		Digit character
\D    		Non-digit character

Repetition Specifiers

*            	0 or more repetitions
+            	1 or more repetitions
?            	1 or 0 repetitions
{n}            	Exactly n repetitions
{n,}        	At least n repetitions
{n,m}        	At least n but not more than m repetitions

Flags

DOTALL, S  	Make . match any character, including newlines
IGNORECASE, I 	Do case-insensitive matches
LOCALE, L 	Do a locale-aware match
MULTILINE, M 	Multi-line matching, affecting ^ and $
VERBOSE, X 	Enable verbose REs.

Search Functions

match()  	Determine if the RE matches at the beginning of the string.
search() 	Scan through a string, looking for any location where this RE matches.
findall() 	Find all substrings where the RE matches, and returns them as a list.
finditer() 	Find all substrings where the RE matches, and returns them as an iterator.

Result Object Functions

group()  	Return the string matched by the RE
start() 	Return the starting position of the match
end() 		Return the ending position of the match
span() 		Return a tuple containing the (start, end) positions of the match

Simple example

>>> import re
>>> src = 'abracadabra'
>>> result = re.search('*ab*', src)
>>> print result.group()
'ab'

Note that if you are going to use the search a lot, you can improve performance by compiling the regex ahead of time using 're.compile(regex)'

Other points of interest

You can use a regex as the basis for a split or substitution, as follows

>>> regex = re.compile(r'\W+')   	#the regex for word boundaries
>>> result = regex.split('This is a test')
>>> print result
['This','is','a','test']