This expression seems to do the job without loopholes: ^([0-9]|[1-9][0-9]|100)$

If we want to permit decimals, then it gets more interesting. I'm no regexpert, but something like this should ensure we get either an integer from 0 to 100 or any decimal in between, with up to 2d.p. (also permitting 100.0 and 100.00!):

^100(\.[0]{1,2})?|([0-9]|[1-9][0-9])(\.[0-9]{1,2})?$

Spot any problems with these regular expressions...? Please comment and help me improve!

Originally posted by Mike Lush, then augmented by Keith Westcott.

You're unlikely to think about this until you need to, but there is no obvious way of searching within a bunch of files for text using a regular expression. I needed to search within thousands of .tab files to find those with that contained a common error in the name of the raster file (all the rasters started with numbers, but some of the tab files lacked them, showing only e.g. File "SP8955.tif"). I needed to find files containing the regular expression File "[A-Za-z][A-Za-z] so that I could correct them by adding the relevant number.