帮忙分析一下这段PHP程序里正则表达式的含义

帮忙分析一下这段PHP程序里正则表达式的含义这段程序是用来分析SQL语句的        $flag = PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY;
        $tokens = preg_split('/(\'(?:(?:\\\\\')|(?:\'\'))*(?:[^\']+(?:(?:\\\\\')|(?:\'\'))*)+\')/', $query, -1, $flag);
        $sql = "";
        foreach ($tokens as $token) {
            if (preg_match('/^\'.*\'$/', $token)) {
                $sql .= $token;
                continue;
            }
            if (strpos($token, '?') !== false) {
                $sub_tokens = preg_split('/(\?)/', $token, -1, $flag);
                foreach ($sub_tokens as $sub_token) {
                    if ($sub_token == '?') {
                        $replace = array_shift($args);
                        if ($replace == null) {
                            trigger_error("The number of place holder token is not equal the number of data", E_USER_ERROR);
                        }
                        mysql_real_escape_string($replace, $this->_connection);
                        $sql .= "'$replace'";
                    }
                    else {
                        $sql .= $sub_token;
                    }
                }
            }
            else {
                $sql .= $token;
            }
        }