Boost regex to capture all repeated patterns - c++

Question

I have a simple text file with the following contents:
VALUE "foo"
ANOTHERVALUE "bar"
YETANOTHERVALUE "barbar"
The values in column 1 are known.
I want to capture everything in column 1 and in column 2.
My solution involves manually writing all the possible values of column 1 (which is known), into the regex string but obviously this is not ideal practice since I am basically repeating code and this does not allow the ordering to be flexible:
const char* re =
"^[[:space:]]*"
"(VALUE)[[:space:]]*\"(.*)\"[[:space:]]*"
"(ANOTHERVALUE)[[:space:]]*\"(.*)\"[[:space:]]*"
"(YETANOTHERVALUE)[[:space:]]*\"(.*)\"[[:space:]]*";

Answer 1

I'm citing commenter Igor Tandetnik here, because he almost gave the complete answer in his comment:
Regular expressions capture exactly as many substrings as there are
left parentheses in the expression [...]
The right way to solve this problem is to write a regex that matches a
single pair, [...]
\s*([a-zA-Z]+)\s*"(.*?)"
Notes:
\s is equivalent to [[:space:]]
.*? is used to stop searching after the 2nd " instead of the last " in the string
and apply it repeatedly, e.g. via std::regex_iterator
The boost equivalent is boost::regex_iterator.
#include <iostream>
#include <string>
#include <algorithm>
#include <boost/regex.hpp>
const boost::regex expr{ R"__(\s*([a-zA-Z]+)\s*"(.*?)")__" };
const std::string s =
R"(VALUE "foo"
ANOTHERVALUE "bar"
YETANOTHERVALUE "barbar"
)";
int main() {
boost::sregex_iterator it{ begin(s), end(s), expr }, itEnd;
std::for_each( it, itEnd, []( const boost::smatch& m ){
std::cout << m[1] << '\n' << m[2] << std::endl;
});
}
Live demo.
Notes:
I'm using raw string literals to make the code cleaner.

Answer 2

I would use a little Spirit Parser here:
Reading Into A Map
Live On Coliru
#include <boost/fusion/adapted/std_pair.hpp> // reading maps
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>
#include <iostream>
#include <fstream>
#include <map>
auto read_config_map(std::istream& stream) {
std::map<std::string, std::string> settings;
boost::spirit::istream_iterator f(stream >> std::noskipws), l;
using namespace boost::spirit::x3;
auto key_ = lexeme [ +upper ];
auto value_ = lexeme [ '"' >> *~char_('"') >> '"' ];
if (!phrase_parse(f, l, -(key_ >> value_) % eol >> eoi, blank, settings))
throw std::invalid_argument("cannot parse config map");
return settings;
}
auto read_config_map(std::string const& fname) {
std::ifstream stream(fname);
return read_config_map(stream);
}
int main() {
for (auto&& entry : read_config_map(std::cin))
std::cout << "Key:'" << entry.first << "' Value:'" << entry.second << "'\n";
}
Prints:
Key:'ANOTHERVALUE' Value:'bar'
Key:'VALUE' Value:'foo'
Key:'YETANOTHERVALUE' Value:'barbar'