Regular expression to validate username

Question

I\'m trying to create a regular expression to validate usernames against these criteria:

1. Only contains alphanumeric characters, underscore an

Answer 1

^(?=.{8,20}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?

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If your browser raises an error due to lack of negative look-behind support, use the following alternative pattern:

^(?=[a-zA-Z0-9._]{8,20}$)(?!.*[_.]{2})[^_.].*[^_.]$