C++ regular expression to match a string - c++

Question

I'm a little poor with regular expressions so I would appreciate help if someone can tell me what the right regular expression would be to capture the three elements that are in this format -
<element1>[<element2>="<element3>"]
I could use boost if needed. The delimiters in this string are '[', '=', ']', '"' and ' '.
Update: This is what I tried till now -
int main(void) {
std::string subject("foo[bar=\"baz\"]");
try {
std::regex re("([a-zA-Z]+)[([a-zA-Z])=");
std::sregex_iterator next(subject.begin(), subject.end(), re);
std::sregex_iterator end;
while (next != end) {
std::smatch match = *next;
std::cout << match.str() << std::endl;
next++;
}
} catch (std::regex_error& e) {
std::cout << "Error!" << std::endl;
}
}
Though this give me -
foo[
bar
baz
Thanks

Answer 1

You don't need iterators for this, you can match it all in one expression with capture groups (<capture>) that return sub matches like this:
// Note: Raw string literal R"~()~" removes the need to escape the string
std::regex const e{R"~(([^[]+)\[([^=]+)="([^"]+)"\])~"};
// ^ 1 ^ ^ 2 ^ ^ 3 ^
// | | | | |_____|------- sub_match #3
// | | | |
// | | |_____|---------------- sub_match #2
// | |
// |_____|------------------------- sub_match #1
std::string s(R"~(foo[bar="baz"])~"); // Raw string literal again
std::smatch m;
if(std::regex_match(s, m, e))
{
std::cout << m[1] << '\n'; // sub_match #1
std::cout << m[2] << '\n'; // sub_match #2
std::cout << m[3] << '\n'; // sub_match #3
}

Answer 2

You could use \[<\[" \]?(\[^<>\[\]" =\x0a\x0d\]+)\[>\[" \]? to get the elements:
#include <string>
#include <sstream>
#include <vector>
#include <iterator>
#include <regex>
#include <iostream>
#include <iomanip>
auto input_text{
R"(foo[bar="baz"]
<element1>[<element2>="<element3>"])"};
auto fromString(std::string str) {
std::vector<std::string> elements;
std::regex r{R"([<\[" ]?([^<>\[\]" =\x0a\x0d]+)[>\[" ]?)"};
std::istringstream iss(str);
auto it = std::sregex_iterator(str.begin(), str.end(), r);
auto end = std::sregex_iterator();
for(; it != end; ++it) {
auto match = *it;
auto element = match[1].str();
elements.push_back(element);
}
return elements;
}
int main()
{
auto result = fromString(input_text);
for (auto t : result) {
std::cout << t << '\n';
}
return 0;
}
Output:
foo
bar
baz
element1
element2
element3
Live demo