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I am using this regex to validate usernames for my app. 3-20 characters, alphanumeric, and underscores. Not beginning with, or ending with an underscore and not having two consecutive underscores.

username.search(/^(?=.{3,20}$)(?![_])(?!.*[_]{2})[a-zA-Z0-9_]+(?<![_])$/)

But is is failing on iOS / safari with this error:

SyntaxError: Invalid regular expression: invalid group specifier name

How can I make this regex work in all major browsers?

Tristan
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3 Answers3

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You could omit the negative lookbehind at the end, but also omit the other 2 negative lookaheads (?![_])(?!.*[_]{2}) and only use the one at the start to check for the length 3 - 20.

Instead of the 2 negative lookaheads, you could match 1+ times [a-zA-Z0-9] so that the string can not start with an underscore.

Then use a repeating pattern to match a single _ followed by again 1+ times [a-zA-Z0-9] so that the string can not end with an underscore.

^(?=.{3,20}$)[a-zA-Z0-9]+(?:_[a-zA-Z0-9]+)+$

See the Regex demo

The fourth bird
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  • How would I update this to not require the underscore, but rather allow it? I tried fiddling with the demo, but couldn't get it right. – Tristan Jul 11 '19 at 02:26
  • @Tristan You could make it optional using a question mark `_?` https://regex101.com/r/fRSqgs/2 – The fourth bird Jul 11 '19 at 07:01
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From my past experience using negative look behind in Javascript is not well supported.

You can see must browsers do not support it in the can I use website. The Regex Pal website also shows an error for this and recommends not to use it.

ps. I didn't have time to create a regex that would work for you.

Trevor Reimer
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I'm guessing that you could simplify and try for instance with an expression similar to:

^(?![_])(?!.*[_]{2})[a-zA-Z0-9_]{2,19}[a-zA-Z0-9]$
Emma
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