This may be a fairly unique occurrence, but here is a regex which takes about a second to fail on my machine, straight out of man.txt:

Code:
//echo -a $regex(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,/(\D+|<\d+>)*[!?]/)


Change it to a possessive quantifier and it fails immediately:

Code:
//echo -a $regex(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,/(\D++|<\d+>)*[!?]/)


Just one example of how a seemingly very similar regex can take a lot longer to compute smile