CODEWITHSUNDEEP
c++regexboost
Laurent D.
Laurent D.

Reputation: 15

C++ boost::regex multiples captures

I'm trying to recover multiples substrings thanks to boost::regex and put each one in a var. Here my code :

unsigned int i = 0;
std::string string = "--perspective=45.0,1.33,0.1,1000";
std::string::const_iterator start = string.begin();
std::string::const_iterator end = string.end();
std::vector<std::string> matches;
boost::smatch what;
boost::regex const ex(R"(^-?\d*\.?\d+),(^-?\d*\.?\d+),(^-?\d*\.?\d+),(^-?\d*\.?\d+))");

string.resize(4);

while (boost::regex_search(start, end, what, ex)
{
    std::string stest(what[1].first, what[1].second);
    matches[i] = stest;
    start = what[0].second;
    ++i;
}

I'm trying to extract each float of my string and put it in my vector variable matches. My result, at the moment, is that I can extract the first one (in my vector var, I can see "45" without double quotes) but the second one in my vector var is empty (matches[1] is "").

I can't figure out why and how to correct this. So my question is how to correct this ? Is my regex not correct ? My smatch incorrect ?

Upvotes: 0

Views: 410

Answers (3)

alphashooter
alphashooter

Reputation: 314

  1. Firstly, ^ is symbol for the beginning of a line. Secondly, \ must be escaped. So you should fix each (^-?\d*\.?\d+) group to (-?\\d*\\.\\d+). (Probably, (-?\\d+(?:\\.\\d+)?) is better.)

  2. Your regular expression searches for the number,number,number,number pattern, not for the each number. You add only the first substring to matches and ignore others. To fix this, you can replace your expression with (-?\\d*\\.\\d+) or just add all the matches stored in what to your matches vector:

 while (boost::regex_search(start, end, what, ex))
 {
      for(int j = 1; j < what.size(); ++j) 
      { 
           std::string stest(what[j].first, what[j].second);
           matches.push_back(stest);
      }
      start = what[0].second;
 }

Upvotes: 1

laune
laune

Reputation: 31290

A repeated search in combination with a regular expression that apparently is built to match all of the target string is pointless.

If you are searching repeatedly in a moving window delimited by a moving iterator and string.end() then you should reduce the pattern to something that matches a single fraction.

If you know that the number of fractions in your string is/must be constant, match once, not in a loop and extract the matched substrings from what.

Upvotes: 0

Sabuj Hassan
Sabuj Hassan

Reputation: 39365

You are using ^ at several times in your regex. That's why it didn't match. ^ means the beginning of the string. Also you have an extra ) at the end of the regex. I don't know that closing bracket doing there.

Here is your regex after correction:

(-?\d*\.?\d+),(-?\d*\.?\d+),(-?\d*\.?\d+),(-?\d*\.?\d+)

A better version of your regex can be(only if you want to avoid matching numbers like .01, .1):

(-?\d+(?:\.\d+)?),(-?\d+(?:\.\d+)?),(-?\d+(?:\.\d+)?),(-?\d+(?:\.\d+)?)

Upvotes: 1

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