[Patch] Change default executor to DFS in regex

[Tim Shen]

As the comment in this patch said, DFS approach is faster in simple
regex, but exponentially slower in complex(many quantifier) cases.

DFA optimization could be added, but I'm afraid it needs rewriting
regex_automaton.*.

Tested under -m32 and -m64.

Thanks!


-- 
Tim Shen
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