Regex for no duplicate characters from a limited character pool

Question

Is there a way to write a regex to match a string that only contains certain characters, and never repeats those characters? I already wrote some code using a set to impleme

Answer 1

I'm answering this question

Visual C++ Regex: Matching no repeating letter

which was marked an exact duplicate of this question.

No repeating of any letter ?
Does that mean consecutive letters, or is abaca ok ?

if not, the regex would be yours modified to this :

^(?=.*[a-z])(?!.*([a-z]).*\1).{4,20}$

Expanded

 ^ 
 (?= .* [a-z] )
 (?!
      .* 
      ( [a-z] )                     # (1)
      .* \1 
 )
 .{4,20} 
 $