What's a pathological regex that blows up many parsers (both in time & memory)? and which parsers? Bonus points the more basic and standard the regex is, and the more likely that a non-malicious user might innocently come up with it. Feel free to post actual time and memory data, and parser version.
(I seem to remember that excessive lookbehind assertions or (EDIT:)backtracking in Perl are said to do this, or at least used to be. Anything else?)
Adapted from the first example in the article Regular Expression Matching Can Be Simple And Fast (but is slow in Java, Perl, PHP, Python, Ruby, ...):
perl -e '$n=29; ("a" x $n) =~ (("a?" x $n).("a" x $n))'
Which takes 40+ seconds on my system. Then do $n++ for exponentially increasing fun...