Find out which group matches in Java regex without linear search?

Question

I have some programmatically assembled huge regex, like this

(A)|(B)|(C)|...

Each sub-pattern is in its capturing group. When I get a match, how do I figure out which group matches without linearly testing each group(i) to see it returns a non-null string?

Answer 1

If your regex is programmatically generated, why not programmatically generate n separate regexes and test each of them in turn? Unless they share a common prefix and the Java regex engine is clever, all alternatives get tested anyway.

Update: I just looked through the Sun Java source, in particular, java.util.regex.Pattern$Branch.match(), and that does also simply do a linear search over all alternatives, trying each in turn. The other places where Branch is used do not suggest any kind of optimization of common prefixes.

Answer 2

You can use non-capturing groups, instead of:

(A)|(B)|(C)|...

replace with

((?:A)|(?:B)|(?:C))

The non-capturing groups (?:) will not be included in the group count, but the result of the branch will be captured in the outer () group.