Question

How to validate a username using php

My rules are:

Username can only contain letters, numbers and 1 underscore.

This is my current preg_match

preg_match('/^[a-zA-Z0-9][_]{1}[a-zA-Z0-9]+$/',$_POST['uname'])

Answer 1

This should do it:

/^[a-zA-Z0-9]*_?[a-zA-Z0-9]*$/

Keep in mind that a character class of a single item is redundant. Write the atom solely instead. And {1} is another seriously redundant quantifier.

/[_]{1}/

is just the same as

/_/

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Edit:

Based on the new constrains and in the helpful comment by chris, this seems to be a better regex:

/^[a-zA-Z0-9]+_?[a-zA-Z0-9]+$/D

It matches usernames of at least two character, not ending or beginning with the optional underscore, and no newlines at the end.