http://qs321.pair.com?node_id=1208849


in reply to Re^4: creating qr from existing regex
in thread creating qr from existing regex

In addition to what AnomalousMonk said. Plain regular expressions can be constructed to be DoS attacks; even unintentionally. The handling varies from Perl version to version and I am pretty sure regular expressions were impossible to interrupt with a timeout historically so the "attack" was impossible to guard against. I have no idea if this is still the case in newer versions or when it changed if it did.