I think this is rather an answer to Re^6: Nonrepeating characters in an RE (performance).

And you tested the "worst case" of a pattern like adieu which resulted in 8632 hits.

And it turns out that my intuition, that a posteriori filtering outside the regex is a sufficient approach, wasn't too bad.

You didn't tell us the Perl version and I can't see a use re 'eval' happening, so no info about the observed slow down with newer versions.

> (but it's important to get that anchor in the right place)

Yes, that's a lesson I had to learn for this task already Re: Merging multiple variations of a serial number (regex as "mini prolog").

Because otherwise the regex will never reach an anchor behind a FAIL, hence longer words will be checked too.

The OP didn't tell us which anchors he plans to use, so ...

There is also (?PRUNE) to be considered to avoid unwanted backtracking, but in this case we have no quantifiers to spawn a tree anyway.

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

In reply to Re^7: Nonrepeating characters in an RE by LanX
in thread Nonrepeating characters in an RE by BernieC

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