private function _parsePairAfterSlash(int &$offset): string
    {
        $idx = $offset;
        if ($idx >= $this->_len) {
            throw new \UnexpectedValueException(
                "Unexpected end of escape sequence.");
        }
        $c = $this->_dn[$idx++];
        // special | \ | " | SPACE
        if (false !== strpos(self::SPECIAL_CHARS . '\\" ', $c)) {
            $val = $c;
        } else { // hexpair
            if ($idx >= $this->_len) {
                throw new \UnexpectedValueException("Unexpected end of hexpair.");
            }
            $val = @hex2bin($c . $this->_dn[$idx++]);
            if (false === $val) {
                throw new \UnexpectedValueException("Invalid hexpair.");
            }
        }
        $offset = $idx;
        return $val;
    }
private function _parsePairAfterSlash(int &$offset): string
    {
        $idx = $offset;
        if ($idx >= $this->_len) {
            throw new \UnexpectedValueException(
                "Unexpected end of escape sequence.");
        }
        $c = $this->_dn[$idx++];
        // special | \ | " | SPACE
        if (false !== strpos(self::SPECIAL_CHARS . '\\" ', $c)) {
            $val = $c;
        } else { // hexpair
            if ($idx >= $this->_len) {
                throw new \UnexpectedValueException("Unexpected end of hexpair.");
            }
            $val = @hex2bin($c . $this->_dn[$idx++]);
            if (false === $val) {
                throw new \UnexpectedValueException("Invalid hexpair.");
            }
        }
        $offset = $idx;
        return $val;
    }
private function _parsePairAfterSlash ( int & $ offset ) : string { $ idx = $ offset ; if ( $ idx >= $ this -> _len ) { throw new \ UnexpectedValueException ( "Unexpected end of escape sequence." ) ; } $ c = $ this -> _dn [ $ idx ++ ] ; // special | \ | " | SPACE if ( false !== strpos ( self :: SPECIAL_CHARS . '\\" ' , $ c ) ) { $ val = $ c ; } else { // hexpair if ( $ idx >= $ this -> _len ) { throw new \ UnexpectedValueException ( "Unexpected end of hexpair." ) ; } $ val = @ hex2bin ( $ c . $ this -> _dn [ $ idx ++ ] ) ; if ( false === $ val ) { throw new \ UnexpectedValueException ( "Invalid hexpair." ) ; } } $ offset = $ idx ; return $ val ; }
Parse 'pair' after leading slash.

@param int $offset
@throws \UnexpectedValueException
@return string
Parse pair after leading slash .