Update:
Another regex cheatsheet that I like a lot (nice formatting and decent descriptive text): http://www.yadudigital.com/products/bw/html/idh_regularexpressions.htm
------------------
Finally, a concise regex tutorial, found in the PHP manual (ereg page). I think the copy and paste lost some of the detail so you might want to go to the source, http://ca2.php.net/manual/en/function.ereg.php.
[anti-spam-email-removed] (11-Jan-2002 06:51)
I must've read 50 regular expression tutorials and understood nothing from each. I finally worked them out through playing around and looking at other people's examples. I wrote this to summarise and clarify things. I find it helps as a quick lookup reference:
^ Start of String
$ End of string
n* Zero or more of 'n'
n+ One or more of 'n'
n? A possible 'n'
n Exactly two of 'n'
n At least 2 or more of 'n'
n From 2 to 4 of 'n'
() Parenthesis to group expressions
(n|a) Either 'n' or 'a'
. Any single character
[1-6] A number between 1 and 6
[c-h] A lower case character between c and h
[D-M] An upper case character between D and M
[^a-z] Absence of lower case a to z
[_a-zA-Z] An underscore or any letter of the alphabet
^.[a-z]_?[0-9]*([1-6]|[a-f])[^1-9]a+$
A string beginning with any two characters
Followed by either 1 or 2 lower case alphabet letters
Followed by an optional underscore
Followed by zero or more digits
Followed by either a number between 1 and 6 or a character between a and f (Lowercase)
Followed by a two characters which are not digits between 1 and 9
Followed by one or more n characters at the end of a string