Disclaimer: This may not fit your purposes ...

A rough brute method would be to match all the REs against a given string and measure the time consumed. This way you'll have a best fit.
read more before flaming...

Actually this doesn't necessarily reveal the best fit, but somehow benchmarks the RE engine.
The point is that if you want to determine the best fit you need to write some sort of a RE engine. But that's why you use one, in order not to write it yourself :-)