In a perfect world, code should be correct and fast. Perl's code used to be wrong and fast. Now it is correct and slow. Correct and slow is generally better than wrong and fast. In time it is likely to speed up again and we'll all be happy.

Unfortunately you began with the rude shock of seeing an amazing slowdown. Therefore while in other circumstances you might agree that you want the right answer, anything below the speed which you were accustomed to is bad.

On the specific optimization that you offer, you're right and wrong. You're right that you can optimize that one regular expression that way and it would be good for that regular expression. But it wouldn't speed up the one that you did want to run. Furthermore adding a check for that special case would slow down the compilation of every other regular expression out there (including the one that you wanted to run). Furthermore you've just added a code path that has potential bugs which might not get caught.

This is not to say that you never want to speed up special cases - of course you do and the regular expression engine has a lot of special tricks. But you have to balance out what is sped up by any one trick against how it slows other people down and causes opportunities for bugs to lurk.

That said, I'd like to point out why the optimization that you point out would not solve your problem. It would tell how to solve a particular expression that you weren't running. The one that you tried to run is different enough that the optimization would probably not run. What you actually would have benefited from is an optimization that says, "Check that there are no backreferences within the RE, then turn on the old special case optimizations." Which might or might not work out to be worthwhile. (And I do not wonder that japhy just chose to turn the optimization off rather than put a test that is that complicated in.)

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