I didn't say it directly but now I will. A regexp on perl's recursive regexp engine can cause it to run out of C stack which then triggers a segfault. That aborts your program. There are patches to perl for versions lie 5.8.4+ (or similar) to either mitigate this or completely work around it. This problem is completely gone in 5.9.4. You could upgrade to that immediately if you wished. It was just released yesterday.

Could you give an example of a regexp that would chew up all the memory on a machine? I'm utterly fascinated by this, as I was unaware you could cause recursion in a regexp.

Or does it have to be like a gig of "(((((((((((((((((((((" to do it?

The examples I'm seeing seem to use (??{ to build lambdas into the regs. I suspect that wouldn't apply if they were compiled at runtime -- ie, without use re eval.

Not all of the memory on the machine, all of the C stack. It's a fixed-size piece of memory reserved for the the storage of the parameters and the local variables of C functions. Perl's guts and functions called via XS make use of this stack.

See the thread What perl operations will consume C stack space? and particularly hv's reply.