in reply to
Re^4: Compiling Regular Expressions
in thread Compiling Regular Expressions
It should also be noted that study() is not as well tested in perl as it ought to be, and as a result changes to the regexp engine over the years have managed to introduce various problems that cause study() not to gain as much speed as it should, and in some cases even give the wrong answer.
In particular, I don't think study() knows anything about Unicode (though I haven't checked).
The plan is that 5.10.0 should involve a big cleanup of the regexp engine, so this may improve in the future.