regex repeated character count

Question

If I have a set of characters like \"abcdefghij\" and using this characters, I generate random a password using this characters. A generated password can have, for example,

Answer 1

You could use something like:

/^
  (?:(.)
     (?!\1)           # neighbor characters are not identical
     (?!(?>.*?\1){2}) # character does not occur more than twice
  )*
\z/x

Perl quoting, the atomic group can be removed if not supported.

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In Java regex it could be written like:

^(?:(.)(?!\1|(?:.*?\1){2}))*\z