What's a simple way to return the first regex match in a string with modern C++?

Question

What is a simple way to return the first part of a string that matches a regex in C++11?

Example: For the string "The great brown fox jumped over the lazy dog." and the regexp /g[a-z]+/ the returned match would be "great"

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@Someprogrammerdude: It looks like I figured out what OP was asking for; although, frankly, it may very well be a dupe. — einpoklum, Jan 30 '17 at 16:01
@Qix: I think Someprogrammerdude's comment was somewhat more helpful than yours... — einpoklum, Jan 30 '17 at 16:07

einpoklum · Accepted Answer · 2017-01-30T16:02:34.437

Is this what you meant?

#include <regex>
#include <iostream>

// ... include first_match() somewhere...

int main()
{
    std::string subject("The great brown fox jumped over the lazy dog.");
    std::string result;
    std::regex re("g[a-z]+");
    std::smatch match;
    if (std::regex_search(subject, match, re)) { std::cout << match.str(0); }
    else { std::cout << "(No matches)"; }
    std::cout << '\n';
    return 0;
}

If you'd like that as a function, I'd allow myself to use std::optional. It's part of C++17, not C++11, but it's already available in C++14 as std::experimental::optional, and for C++11 you can use Boost's boost::optional) instead. Having it available, you would write something like this:

std::optional<std::string> first_match(
    const std::string&  str, 
    const std::regex&   expression)
{
    std::smatch match;
    return std::regex_search(str, match, expression) ?
        std::optional<std::string>(match.str(0)) : std::nullopt;
}

which you can use as follows:

#include <regex>
#include <iostream>
#include <optional>

int main()
{
    std::cout <<
        first_match(
            "The great brown fox jumped over the lazy dog.", "g[a-z]+"
        ).value_or("(no matches)") << '\n';
    return 0;
}

What's a simple way to return the first regex match in a string with modern C++?

1 Answers1