A too smart RegEx engine would already break the (?{}) part of the code, which is evaluated every time the engine runs over it. The main problem is that (?{}) is an experimental feature which may be changed or deleted in future Perl versions. Still, AFAIK, it is considered useful for some RegExes (the above one is fairly standard) which will hopefully prevent major changes in the syntax. And don't forget we have Perl 6 coming up ;-).
Cheers,
CombatSquirrel.

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Entropy is the tendency of everything going to hell.