I never worked with regular expressions before. I heard how great they can be, but never really knew. Regular expressions are really cool.
RegEx Sample: Number with 0 to 3 decimal places (|\-)\d+(.\d{3}|.\d{2}|.\d{1}|)
Regular expressions look pretty complex and sort of cryptic at first glance, but if you have a good tutorial to go through it one part at time they aren't really difficult to learn how to write.
This site has some good tutorials on Regular Expressions: http://www.regular-expressions.info/tutorialcnt.html
Happy RegEx-ing!!