Hi

( DISCLAIMER this is a theoretical question about pure regexes, I know how to solve this in Perl, it's a follow up to this thread)

I'm banging my head at this problem, how do I match strings where at least one character happens exactly once, with a pure regex?

(i.e. without additional Perl code, especially embedded one, and without manipulating the input)

Finding all characters which aren't repeated afterwards is easy with a lookahead assertion

  DB<200> p
bab

  DB<200> x / (.) (?! .* \1 ) /gx
0  'a'
1  'b'

[download]

but combining with a lookbehind fails, b/c variable length is not permitted

  DB<211> x / (.) (?<! \1 .* ) /gx
Variable length lookbehind not implemented in regex m/ (.) (?<! \1 .* 
+) / at (eval 261)[C:/Perl_64/lib/perl5db.pl:646] l
ine 2.

[download]

(actually already using the backreference \1 fails, since the placeholder has variable length)

So it boils down to the question:

All workarounds I found so far only work with ugly cheats, like hardcoding all cases for a fixed length string only.

I think it might be possible with recursive regexes and relative backreferences, but still ...

Cheers Rolf
(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!

In reply to Regex: matching character which happens exactly once by LanX

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