filter sanitize_user

Фильтр sanitize_user нужно дополнить фильтром validate_username, примерно так:

function my_sanitize_user( $username, $raw_username, $strict ) {
	$username = wp_strip_all_tags( $raw_username );
	$username = remove_accents( $username );
	// Kill octets.
	$username = preg_replace( '|%([a-fA-F0-9][a-fA-F0-9])|', '', $username );
	// Kill entities.
	$username = preg_replace( '/&.+?;/', '', $username );

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

	$username = trim( $username );
	// Consolidate contiguous whitespace.
	$username = preg_replace( '|\s+|', ' ', $username );

	// Remove some leading and trailing characters.
	$username = ltrim( $username, '-_.' );
	$username = trim( $username, '-_.' );

	return $username;
}
add_filter( 'sanitize_user', 'my_sanitize_user', 10, 3 );

function my_validate_username( $valid, $username ) {
	$sanitized = sanitize_user( $username, true );

	$edited_username = ltrim( $username, '-_.' );
	$edited_username = trim( $edited_username, '-_.' );

	$valid = ( $sanitized == $edited_username && ! empty( $sanitized ) );

	return $valid;
}
add_filter( 'validate_username', 'my_validate_username', 10, 2 );