function bp_activity_get_userid_from_mentionname( $mentionname ) {
	$user_id = false;

	/*
	 * In username compatibility mode, hyphens are ambiguous between
	 * actual hyphens and converted spaces.
	 *
	 * @todo There is the potential for username clashes between 'foo bar'
	 * and 'foo-bar' in compatibility mode. Come up with a system for
	 * unique mentionnames.
	 */
	if ( bp_is_username_compatibility_mode() ) {
		// First, try the raw username.
		$userdata = get_user_by( 'login', $mentionname );

		// Doing a direct query to use proper regex. Necessary to
		// account for hyphens + spaces in the same user_login.
		if ( empty( $userdata ) || ! is_a( $userdata, 'WP_User' ) ) {
			global $wpdb;
			$regex   = esc_sql( str_replace( '-', '[ \-]', $mentionname ) );
			$user_id = $wpdb->get_var( "SELECT ID FROM {$wpdb->users} WHERE user_login REGEXP '{$regex}'" );
		} else {
			$user_id = $userdata->ID;
		}

	// When username compatibility mode is disabled, the mentionname is
	// the same as the nicename.
	} else {
		$user_id = bp_core_get_userid_from_nicename( $mentionname );
	}


	return $user_id;
}