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

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