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

Answer 2

AFAIK, this cannot be done with a simple regexp (particularly, ensuring that a letter only appears twice at max. You could do a bunch of expressions like

[^a]*(a[^a]*(a[^a]*))
[^b]*(b[^b]*(b[^b]*))
....

and also (matching means the validation failed):

[^a]*aa[^a]*
[^b]*bb[^b]*

but obviously this is not good idea.

The condition that the characters do not repeat together maybe can treated with capturing groups, but I am almost sure the other one cannot be checked with regex.

BTW... why the obsession with regex? Programming these checks are trivial, regex is useful in a set of cases but not every check can be done with regex.