|Problems? Is your data what you think it is?|
Re: slow regular expressionsby blokhead (Monsignor)
|on Aug 08, 2006 at 05:37 UTC||Need Help??|
As GrandFather said, your regex is equivalent to /a*b/, so it's easy to see that the regex can never match. The slowdown is because of the backtracking done by Perl's regex engine.
Let's look at a smaller example:
Except I'll make the following changes:
So you can see that it tries a way to fill $1 and $2 (and $`), fails to match /(?!)/, backtracks, and tries again repeatedly. It tries every possible way before the regex match operation finally returns with a failure. In this small example it tries 35 combinations before eventually failing.
Now if you make the string a lot longer, and give it more ways for the prefixes to match (more /a*/ regexes to fill), it will take a whole lot longer. By my calculations, the regex you gave above will try
combinations before eventually failing. Even at a billion combinations per second, that's still 782 million years ;)
1: If I left /b/ as the last part of the regex instead of /(?!)/, then the regex engine seemed to optimized away my print statement.
Update: revised my big number calculation, not that it makes a huge difference. It's 61+33 choose 33 if you're curious. That's the number of ways to put 61 balls (the a's) into 34 ordered bins (32 /a*/ regexes, plus $` and $').