Mine was implementing negative lookaround assertions with plain grep -E. I did it for detecting cross-posts between certain newsgroups. The precise regex I used was anchored to the beginning of the line:

I even made some pseudo-algorithm for building more:

^
([^e]|e
([^s]|s
([^\.]|\.
([^c]|c

<blah>

([^n]|n
([^u]|u
[^x]
)))))))))))))))
[download]

But didn't bother to automate it. In fact, I've never used this again.