You're also neglecting stuff like 12.. Here are a few steps to getting a regex like you want:

/^\d+$/                      # integer
/^\d+(?:\.\d+)?$/            # integer or optional real
/^\d+(?:\.\d\d)?$/           # int or opt. 2-digit float
/^\d+(?:\.(?:\d\d)?)?$/      # int, int with opt '.', or 2-digit float

# Ways of doing optional integer portion (e.g. .12)
/^\d*(?:\.(?:\d\d)?)?$/           # WRONG (matches "")
/^\d+(?:\.(?:\d\d)?)?$|^\.\d\d$/  # better, but still ugly
[download]

Forget about doing gas prices with a regex like this, though (e.g. 1.499). Don't forget about negative-signs also, if that's necessary.

I also haven't tested any of that.