title: Pathological regex that blows up (time & memory)?
body: <p>What's a pathological regex that blows up many parsers (both in time &amp; 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.</p>
  
  <p>(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?)</p>