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

^(?:[0-9A-Za-z]+|([._%+-])(?!\1))+$

Broken down:

• (?: … )+ — one or more of either:
  • [0-9A-Za-z]+ — one or more alphanumeric characters or
  • ([._%+-]) — any allowed non-alphanumeric
    • (?!\1) — which isn't followed by the exact same character

Allows:

• foo
• foo.+bar
• -700.bar+baz

Disallows:

• foo..bar
• foo.+bar--baz

It works by capturing the matched non-alphanumeric characters into the first backreference (\1) each time the outer, not capturing group is matched and using a negative look-ahead ((?! … )) to make sure the same character doesn't appear twice in a row. Be aware that not all regex flavors support negative look-ahead!