Determining the location of C++11 regular expression matches - c++

Question

How do I efficiently determine the location of a capture group inside a searched string? Getting the location of the entire match is easy, but I see no obvious ways to get at capture groups beyond the first.
This is a simplified example, lets presume "a*" and "b*" are complicated regexes that are expensive to run.
#include <iostream>
#include <regex>
#include <string>
using namespace std;
int main()
{
regex matcher("a*(needle)b*");
smatch findings;
string haystack("aaaaaaaaneedlebbbbbbbbbbbbbb");
if( regex_match(haystack, findings, matcher) )
{
// What do I put here to know how the offset of "needle" in the
// string haystack?
// This is the position of the entire, which is
// always 0 with regex_match, with regex_search
cout << "smatch::position - " << findings.position() << endl;
// Is this just a string or what? Are there member functions
// That can be called?
cout << "Needle - " << findings[1] << endl;
}
return 0;
}
If it helps I built this question in Coliru: http://coliru.stacked-crooked.com/a/885a6b694d32d9b5

Answer 1

I will not mark this as and answer until 72 hours have passed and no better answers are present.
Before asking this I presumed smatch::position took no arguments I cared about, because when I read the cppreference page the "sub" parameter was not obviously an index into the container of matches. I thought it had something to do with "sub"strings and the offset value of the whole match.
So my answer is:
cout << "Needle Position- " << findings.position(1) << endl;
Any explanation on this design, or other issues my line of thinking may have caused would be appreciated.

Answer 2

According to the documentation, you can access the iterator pointing to the beginning and the end of the captured text via match[n].first and match[n].second. To get the start and end indices, just do pointer arithmetic with haystack.begin().
if (findings[1].matched) {
cout << "[" << findings[1].first - haystack.begin() << "-"
<< findings[1].second - haystack.begin() << "] "
<< findings[1] << endl;
}
Except for the main match (index 0), capturing groups may or may not capture anything. In such cases, first and second will point to the end of the string.
I also demonstrate the matched property of sub_match object. While it's unnecessary in this case, in general, if you want to print out the indices of the capturing groups, it's necessary to check whether the capturing group matches anything first.