Comment by patrec

Adding lookbehinds to the regexp library is a terrible idea.

> The regexp implementation provided by this package is guaranteed to run in time linear in the size of the input.

Python's is exponential, because it inherits all the non-regular "regular" expression mess (such as lookbehind and backrefs) from perl.

One would assume esr would have marinated in unix culture for long enough to be aware of this.