It's a thing: https://en.wikipedia.org/wiki/ReDoS https://en.wikipedia.org/wiki/ReDoS And it really happens. StackOverflow had a ~30 minute outage a couple yea

It's a thing: https://en.wikipedia.org/wiki/ReDoS https://en.wikipedia.org/wiki/ReDoS And it really happens. StackOverflow had a ~30 minute outage a couple years ago because of it: http://stackstatus.net/post/147710624694/outage-postmortem-july-20-2016 http://stackstatus.net/post/147710624694/outage-postmortem-j... One of the key points here is that it's not always obvious from looking at a regex whether it will exhibit catastrophic backtracking (at least, to an untrained eye). Even more insidious, catastrophic backtracking may only happen on certain inputs, which means that tests won't necessarily track it either. Therefore, desiring a performance guarantee here is certainly not "strange." The bottom line is that the various implementation strategies for regex engines have trade offs. On Internet forums, it's popular to "proclaim" for one side or the other. The backtracking advocates like to believe that catastrophic cases never happen or are easily avoided and the linear time advocates like to believe that so long as matching doesn't take exponential time, then it will be fast enough. :-)