I can see validate_user() calls sanitize_user() with strict argument as true, which means uppercase letters are not allowed too.

In source code, I found this comment

// If strict, reduce to ASCII for max portability.
    if ( $strict )
        $username = preg_replace( '|[^a-z0-9 _.\-@]|i', '', $username );

I would like to know how does this affect portability?

  • The i modifier in the regex pattern makes it case-insensitive, just something to note.
    – t31os
    Apr 8, 2011 at 14:58
  • so what does this regex actually do here?
    – Ashfame
    Apr 9, 2011 at 19:56
  • 1
    Replaces anything not a-z(upper or lower), 0-9(ie. any number), an underscore, period, hyphen or an @ symbol..
    – t31os
    Apr 9, 2011 at 23:21
  • so how does it block the uppercase alphabets then? :S
    – Ashfame
    Apr 10, 2011 at 14:25
  • 1
    @Ashfame: actually, the function doesn't block uppercase letters. In the comment above the functions, WP writes: "If $strict is true, only alphanumeric characters (as well as _, space, ., -, * @) are returned.". That means uppercase letter are accepted. In fact, I have a WP where I have the username with uppercase letter, and no problems at all.
    – Anh Tran
    Jun 7, 2011 at 13:44

3 Answers 3


Uppercase characters are not blocked in usernames on single site setups.

Uppercase characters ARE blocked in usernames on multisite setups. The wpmu_validate_user_signup function forces lowercase a-z and numbers 0-9 only.


If You are not allowing users to register site, you may disable 'uppercase' restriction from MS install, just add into your theme functions.php file:

remove_action( 'sanitize_user', 'strtolower' );

Off hand, I suppose it would be so you can use validate_user's results in a URL or a $_GET string. I'm sure there are plenty of other reasons as well, I guess I don't know what they mean by max portability...

  • yes, I am interested in knowing about how this affects portability. anything else like a variable from URL can be configured to converted to lowercase and then made it work. A bit extra work, but will work.
    – Ashfame
    Apr 8, 2011 at 12:27

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