Re: Documentation on debugging regexp performance

[Alexis]

Yuri Khan <address@hidden> writes:

> > I'm running into a surprising regular expressions issue. I have 
> > attached a file (~50k) in which (re-search-forward " +[^:=]+ 
> > +:=?")  seems to be extremely slow. (I killed it after 30 
> > seconds). Truncating the file to its first 20 lines reduces the 
> > time for re-search-forward to about a second, which is still 
> > extremely slow.
>
> I’m no expert on the Emacs regexp implementation, but this part 
> is ambiguous: "[^:=]+ +". The engine will have to backtrack at 
> least once because the first part will greedily slurp all 
> spaces, then the second part will not match. You might want to 
> add the space to the exclusion character class: "[^:= ]+ +".

More generally, i highly recommend Jeffrey Friedl's book 
"Mastering Regular Expressions". It's not Emacs-specific, but it 
provides in-depth explanations of why certain regexen are time- 
and/or space-hungry.


Alexis.