bp_xprofile_validate_nickname_value( $retval,  $field_id, string $value, string $user_id = null )

function bp_xprofile_validate_nickname_value( $retval, $field_id, $value, $user_id = null ) {
	if ( $field_id != bp_xprofile_nickname_field_id() ) {
		return $retval;
	}

	if ( $retval ) {
		return $retval;
	}

	$value = strtolower( $value );
	$field_name = xprofile_get_field( $field_id )->name;

	// Empty nickname
	if ( '' === trim( $value ) ) {
		return sprintf( __( '%s is required and not allowed to be empty.', 'buddyboss' ), $field_name );
	}

	// only alpha numeric, underscore, dash
	if ( ! preg_match( '/^([A-Za-z0-9-_\.]+)$/', $value ) ) {
		return sprintf( __( 'Invalid %s. Only "a-z", "0-9", "-", "_" and "." are allowed.', 'buddyboss' ), $field_name );
	}

	// cannot have 2 continued special characters
	if ( preg_match( '/([-_\.]{2})/', $value ) ) {
		return sprintf( __( '"-", "_" and "." cannot be repeated twice in %s.', 'buddyboss' ), $field_name );
	}

	// must be shorter then 32 characters
	$nickname_length = apply_filters( 'xprofile_nickname_max_length', 32 );
	if ( strlen( $value ) > $nickname_length ) {
		return sprintf( __( '%s must be shorter than %d characters.', 'buddyboss' ), $field_name, $nickname_length );
	}

	global $wpdb;
	$where = [
		'meta_key = "nickname"',
		'meta_value = "' . $value . '"'
	];

	if ( $user_id ) {
		$where[] = 'user_id != ' . $user_id;
	}

	$sql = sprintf(
		'SELECT count(*) FROM %s WHERE %s',
		$wpdb->usermeta,
		implode( ' AND ', $where )
	);

	if ( $asdf = $wpdb->get_var( $sql ) > 0 ) {
		return sprintf( __( '%s has already been taken.', 'buddyboss' ), $field_name );
	}

	return $retval;
}