Regexp help

Hi,
I'm using the Commons-Validator framework that uses Regexp for masking user input. I'm not familiar with regular expressions and I would like to learn some, but I have a short timeframe to learn them. What online sources would you recommend that briefly introduce one to regular expressions?

Also, if you could give me regular expression examples for the below cases I would really appreciate it:

1) XXXXX-XXXX where X is any digit. This is supposed to be a zip/postal code but the last 4 zip code digits are not required.

2) YYYY...YYY where Y is alpha-numeric and does not include any punctuation marks. Basically I want validate user input for choosing a password that includes only certain set of characters.

Thank you very much,
Mete