Regex non-consecutive chars

Question

Currently I have:

[A-Za-z0-9._%+-]

This matches any string that contains letters, numbers, and certain special chars (._%+-)

Answer 1

Using PHP's PCRE, you can do this:

/^([A-Za-z0-9]|([._%+-])(?!\2))*$/

The \2 is the back-reference that's required to detect a duplicate usage of the same symbol. I'm not sure it's possible to do this without a forward assertion and a back-reference, so there is my working regex tested against:

'foo'         => true,
'bar.baz'     => true,
'bar.biz.buz' => true,
'bar.+bar'    => true,
'bar..bar'    => false,
'biz.baz..'   => false,
'..++..'      => false,
'.faf.'       => true,