title: How to hook validate_username (or sanitize_user) in WP?
body: <p>I am new to wordpress and I am just learning it.
  <strong>How to hook validate_username or sanitize_user functions?</strong></p>
  
  <p>I do not want to call them at all, I want to change them for my particular plugin.</p>
  
  <p>Here is the full story:
  My problem is that I do not know how to change existing WP functions correctly without modifying them (some hooks or something like this).</p>
  
  <p>I have the following problem:</p>
  
  <ul>
  <li>I have a plugin theme-my-login that shows registration form</li>
  <li>it has register_new_user (that handles registering a new user) -> it is not wp standard function, but is written in plugin</li>
  <li><p>this function calls WP standard validate_username:</p>
  
  <pre><code>function register_new_user( $user_login, $user_email,$term, $country  ) {
  $errors = new WP_Error();
  
  $sanitized_user_login = sanitize_user( $user_login );
  $user_email = apply_filters( 'user_registration_email', $user_email );
  
  // Check the username
  if ( $sanitized_user_login == '' ) {
      $errors-&gt;add( 'empty_username', __( '&lt;strong&gt; ط®ط·ط£ &lt;/strong&gt;...', 'theme-my-login' ) );
  } elseif ( !**validate_username**( $user_login ) ) { //HERE WE CALL WP VALIDATE_
      $errors-&gt;add( 'invalid_username', __( '&lt;strong&gt; ط®ط·ط£ &lt;/strong&gt;: 
  </code></pre>
  
  <p>....</p></li>
  </ul>
  
  <p>So, the problem here is that *validate_username* doesn't allow usernames to be in arabic language. But this is what client wants.
  So, I believe I should overwrite or somehow modify existing WP validate_username or sanitize_user (both are in wp-includes/user.php).</p>