regex_match giving unexpected results - c++

Question

I'm trying to write a recursive descent parser and am trying to search a match a regex within a string inputted by the user. I am trying to do the following to try to understand the <regex> library offered by C++11, but I'm getting unexpected results.
std::string expression = "2+2+2";
std::regex re("[-+*/()]");
std::smatch m;
std::cout << "My expression is " << expression << std::endl;
if(std::regex_search(expression, re)) {
std::cout << "Found a match!" << std::endl;
}
std::regex_match(expression, m, re);
std::cout << "matches:" << std::endl;
for (auto it = m.begin(); it!=m.end(); ++it) {
std::cout << *it << std::endl;
}
So based on my regular expression, I expect it to output
Found a match!
matches:
+
+
However, the output I get is:
My expression is 2+2+2
Found a match!
matches:
I feel like I'm making a stupid mistake, but I can't seem to figure out why there's a discrepancy between the outputs.
Thanks,
erip

Answer 1

You've got a few issues. First, let's look at some working code:
#include <regex>
#include <iostream>
int main() {
std::string expr = "2+2+2";
std::regex re("[+\\-*/()]");
const auto operators_begin = std::sregex_iterator(expr.begin(), expr.end(), re);
const auto operators_end = std::sregex_iterator();
std::cout << "Count: " << std::distance(operators_begin, operators_end) << "\n";
for (auto i = operators_begin; i != operators_end; ++i) {
std::smatch match = *i;
std::cout << match.str() << "\n";
}
}
Output:
Count: 2
+
+
Issues with your code:
regex_match() returns false.
You don't have any capturing groups in the regular expression. So even if regex_match() returned true, it wouldn't capture anything.
The number of captures in a regex_match can be determined strictly by looking at the regular expression. So my re will capture exactly one group.
But we want to apply this regular expression on our string multiple times, because we want to find all of the matches. The tool for that is regex_iterator.
We also needed to escape the - in the regular expression. The minus has a special meaning within character classes.

Answer 2

I found the regex_iterator, which solved the problem. Here's the working code:
std::regex re("[-+*/()]");
std::smatch m;
std::cout << "My expression is " << expression << std::endl;
if(std::regex_search(expression, re)) {
std::cout << "Found a match!" << std::endl;
}
try {
std::sregex_iterator next(expression.begin(), expression.end(), re);
std::sregex_iterator end;
while (next != end) {
std::smatch match = *next;
std::cout << match.str() << "\n";
next++;
}
} catch (std::regex_error& e) {
// Syntax error in the regular expression
}