That was just one of the metrics that I could think of... unless the number of regexes were so large that you couldn't rank them yourself (Going with the assumption that I know more about the process than a regex does)

If I had to go completely on just odds of matching, I would think it'd be easiest to take a representative sample of inputs, and test them against each of the regexes, and build a table with the odds.

If you don't have a log of those inputs for testing, then we'd have to get more creative ... I might use something like the following --

• Any character or zero width assertion gets 1 point. (unless the assertion is pointless, like '\W\b\w'
• Any character class of n characters gets f(n) points, where f(n) yields a number less than one, and decreases as n increases (maybe 1/n, or sqrt(1/n) )
• Quantifiers reduce the value of the items they modify ... perhaps as multipliers... ( ? = 0.5; + = 0.6; * = 0.25; +? = 0.7; *? = 0.35 ) (I'm just pulling numbers out of the air...you'd want to tweek the numbers 'till you get good results for your situation).
• Alterations provide something less than the points value of each of its possibilities. (I have no clue on a formula for this one...)

I'm not aware of a module to do this sort of things, but that doesn't mean that there isn't one out there.