Something I learned from a recent answer I gave is you can use zero-width negative lookahead to introduce assertion into your regular expressions.
Try this regular expression:
(?!^[a-zA-Z]+$)(?!^[0-9]+$)^(?:[a-zA-Z0-9]+)$
For an explanation see this answer:
http://www.codeproject.com/script/Answers/Post.aspx?aid=469314[
^].
The regular expression above will forbid strings being made of only digits or only of characters, but will gladly accept a mixture of both.
In my previous solution I also had to use quantifiers, but you've not mentioned any limits on the length of the validated string, so I left it out. You can easily see how this is done from the solution I pointed you to.
Regards,
— Manfred