personally I'm not big into regex but there are regex expressions that will strip out non numeric characters. however the prolem there is guaranteeing that what is left behind is what you wanted.
if you know the string format... eg "level XX" then you have other options, You can split the text string using TOKENAT, or you can split using LEFT, MID, RIGHT. Lots of options. once you have the string subset, use INT (string) to return a numeric value.
reading the manual helps