Well, let's take this guy apart. We know how the leading -? will be treated, so let's leave it out. Then we get two paths that this regex can follow:

(?=\d)\d*\.?\d+$

and

(?=\.\d)\d*\.?\d+$

If the regex takes the first path, we're guaranteed to get at least one digit. This digit must be present in the \d* or \d+ if it goes to the \d*, then we're guaranteed to match the equivalent of

\d+\.?\d+

which will match anything that's comprised of just numbers, and any two numbers with a single period between them.

if it's in the \d+, then we know that there are no periods in the string, since we must have passed through the optional \d* and \.?

so this first path will always match a number number with no fractional portion, or a fractional number with a mandatory leading whole number portion

Now in the other path: (?=\.\d)\d*\.?\d+$

If this regex matches, we're guaranteed to have a period then a number. Since this is a lookahead, that means we've seen past the following \d*, since we can't possibly have picked up a number prior to the period.

So the actual regex in this path is \.\d+, which clearly just matches a number with no preceding 0.

So in any case, I don't think it will match anything other than numbers, as japhy suggests.