Question

I am trying to parse some text using boost. i have looked at the example on the official site but it doesn’t seem to be working. I have written my own example; it doesn’t enter the while loop at all.

#include <boost/regex.hpp>
using namespace std;

int main(void)
{
    string s;
    string::const_iterator st, en;
    s = "int_node:0 int_node:1 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:2 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:3 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:4 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:5 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:6 link_latency:1 bw_multiplier:16 link_weight:1\n"
       "int_node:0 int_node:7 link_latency:1 bw_multiplier:16 link_weight:1";

    st = s.begin();
    en = s.end();
    boost::regex expression("int_node:([0-9]+) int_node:([0-9]+)"
                            " link_latency:([0-9]+) bw_multiplier:([0-9]+)"
                            " link_weight:([0-9]+) long_range:([0-9]+)"
                        " no_of_vcs:([0-9]+)");

    boost::match_results<std::string::const_iterator> what;


    boost::match_flag_type flags = boost::match_default;

    while(boost::regex_search(st, en, what, expression, flags))
    {
      st = what[0].second;
          flags |= boost::match_prev_avail;
          flags |= boost::match_not_bob;

      cout << what[1] << " " << what[2] << " " << what[3] << "\n";
    }

    return 0;
}

I am trying to parse a file that looks like this:

int_node:0 int_node:1 link_latency:1 bw_multiplier:16 link_weight:1
int_node:0 int_node:2 link_latency:1 bw_multiplier:16 link_weight:1
int_node:0 int_node:3 link_latency:1 bw_multiplier:16 link_weight:1
int_node:0 int_node:4 link_latency:1 bw_multiplier:16 link_weight:1

and retrieve the integer values for each field.

Answer 1

Your regular expression has 7 elements but each line you’re searching only has 5.

You could try one of two options. Either reduce the number of elements to only 5

    boost::regex expression("int_node:([0-9]+) int_node:([0-9]+) link_latency:([0-9]+) bw_multiplier:([0-9]+) link_weight:([0-9]+)");

or, if there will be some lines that might have the last two elements (and you want to reference them), use a positive look-ahead assertion like this:

    boost::regex expression("int_node:([0-9]+) int_node:([0-9]+) link_latency:([0-9]+) bw_multiplier:([0-9]+) link_weight:([0-9]+)(?= long_range:([0-9]+) no_of_vcs:([0-9]+))?");

If you’re not familiar with regex, the (?:REGEX)? will say that the last two elements are optional, and the ?: at the beginning means this particular grouping doesn’t count. If there is a value in the 6th column (long_range), then what[6] will contain the long_range value, otherwise it will be blank.