Regex multiple lookaheads not working when both are triggered

user_15340908 0
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I am creating a username field with Laravel and I'm using RegEx to set the formatting rules.
The rule is: Not only numbers **AND OR** underlines + only numbers, chars, and underlines are allowed in general.

Now I came up with this RegEx syntax:
(?!^\d+$)^.[a-zA-Z0-9_]+$

This works fine until the point where I input stuff like:
____2348734

How can I prevent that? Thanks in advance!

laravel regex-lookarounds php regex
1 Answers

You can use either of

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

See the regex demo. Also, consider using \z instead of $ to disallow a trailing newline input.

Details:

  • ^ - start of string
  • (?![\d_]+$) - the string cannot only contain digits / underscores
  • [a-zA-Z0-9_]+ / \w+ - one or more alphanumeric or _ chars
  • $ - end of string. (\z will match the very end of string.)
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