Regex that does not allow consecutive dots

Question

I have a Regex to allow alphanumeric, underscore and dots but not consecutive dots:

^(?!.*?[.]{2})[a-zA-Z0-9_.]+$

I also need to now allow dots in the first and last character of the string.

How can I do this?

Answer 1

You can use it like this with additional lookaheads:

^(?!\.)(?!.*\.$)(?!.*?\.\.)[a-zA-Z0-9_.]+$

• (?!\.) - don't allow . at start
• (?!.*?\.\.) - don't allow 2 consecutive dots
• (?!.*\.$) - don't allow . at end

Answer 2

Re-write the regex as

^[a-zA-Z0-9_]+(?:\.[a-zA-Z0-9_]+)*$

or (in case your regex flavor is ECMAScript compliant where \w = [a-zA-Z0-9_]):

^\w+(?:\.\w+)*$

See the regex demo

Details:

• ^ - start of string
• [a-zA-Z0-9_]+ - 1 or more word chars
• (?:\.[a-zA-Z0-9_]+)* - zero or more sequences of:
  • \. - a dot
  • [a-zA-Z0-9_]+ - 1 or more word chars
• $ - end of string