that's tricky with a regex, because you need to hack a negative lookahead for each position

  DB<130> @a = ("abc","aab","abb","aba") 

  DB<131> for (@a) { say "$_ : $1,$2" if /^(.)(?!.*\1)(.)(?!.*\2)/ }
abc : a,b

  DB<132>

[download]

There are advanced features for repeating patterns where you don't need to know the length in advance. (see perlretut )

But I'd rather prefer a simple split and %seen-hash solution, which is far easier to maintain.

update

Wait ... please try (and test!!!) this solution with relative backrefrences:

  DB<142> @a = ("abc","aab","abb", "aba","abcd","abca" )

  DB<143> for (@a) { say "$_" if /^ ( (.) (?!.*\g-1) )+ $/x }
abc
abcd

  DB<144>

[download]

update

still this regex solution will have O(n^2), while a %seen hash solution can do in O(n)

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

In reply to Re: Nonrepeating characters in an RE (updated) by LanX
in thread Nonrepeating characters in an RE by BernieC

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