Turns out \2 or \g{-1} is not the point here, while the latter is far more stable when refactoring.

Consider this input: "abbcca"

Your recursive approach without an outer loop will produce "aa" while the solution is ""

This might (rather not) be a solution without loops.

  $s =~ s/
       (
         (
           (.) \g{-1} 
         )+
       |
           (.) (?1) \g{-1}
       )
     //xg;

[download]

Frankly I don't know, it's only passing all the tests so far.

Regarding your performance question: the recursive search does a horrible amount of backtracking trying to find a match.

Because every single character allows you to descend further and further, and with longer strings it'll take a while till you mostly don't find the counterpart, and have to track back.

Try to insert various (?{perl snippets}) in your regex and you'll see how many useless branches are searched.

At the same time the original algorithm just needs to compare to neighboring letters, and is hence far faster.

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

In reply to Re: Recursive sub-pattern spectacularly slow, what's wrong? (me, or just this use case, or a bug?) by LanX
in thread Recursive sub-pattern spectacularly slow, what's wrong? (me, or just this use case, or a bug?) by Anonymous Monk

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