My only restrictions are $username needs to have 3-16 letters/numbers and nothing else.

Your code fragment also appears to want to downshift upper-case to lower-case. Is this also a requirement?

If you're trying to verify that a tainted input is of a correct form, you might want to consider providing the user with an appropriate error message for malformed input. See this recent reply to a very similar question, which focuses on providing the user with a reasonable description of what they did wrong.

In reply to Re: a terribly basic question showing my inept skills with regular expressions by dws
in thread a terribly basic question showing my inept skills with regular expressions by jcpunk

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