Mitchell, Neil wrote:
Hi Martijn,
It's not that tricky if you do a regular expression state machine yourself, but that's probably a bit too much work. One way to get a speed up might be to take the regular expressions a,b,c,d and generate a regex a+b+c+d, and one a+b. You can then check any string s against a+b+c+d, if that matches check a+b, if that matches check a. At each stage you eliminate half the regular expressions, which means a match will take log n, where n is the number of regular expressions.
This assumes the underlying regular expression engine constructs a finite state machine, making it O(m) where m is the length of the string to match.
If you're implementing the machine yourself, you can implement it as trie automaton which has some "value" associated with each final state. That is, rather than just accepting or rejecting, when the automaton accepts it returns the value associated with the particular final state that it accepted by. This is a trivial extension on DFA/NFA implementations and is perfectly suited to the problem of combining multiple linear tries (sic: regexes) into a single machine. The minimization algorithms are a bit more complex than for DFA/NFAs, but still fairly straightforward if you're only doing prefix merging. And this gets rid of the O(log n) factor. -- Live well, ~wren