Indeed. To support backreferences, "regex" libraries are forced to use algorithms that can be very slow in the worst case.

The sad thing is that the libraries use the same algorithms even if the expression doesn't contain backreferences. A while ago, Stack Overflow had a brief outage because of regular expression performance, although the expression that caused it didn't even use backreferences or other non-regular features:

http://stackstatus.net/post/147710624694/outage-postmortem-j...

In contrast, Google Code Search - when it still existed - supported regular expression searches over world's public codebases. One key ingredient making this possible was to only use proper regular expressions:

https://swtch.com/~rsc/regexp/regexp4.html