Regular Expressions (RegEx) give me a headache most of the time. That’s where cheat sheets like these com in handy:
http://www.ilovejackdaniels.com/cheat-sheets/regular-expressions-cheat-sheet/
http://regexlib.com/CheatSheet.aspx
http://www.funduc.com/regexp.htm
http://krijnhoetmer.nl/stuff/regex/cheat-sheet/
http://opencompany.org/download/regex-cheatsheet.pdf