std::sub_match

Defined in header `<regex>`
template< class BidirIt > class sub_match;		(since C++11)

The class template std::sub_match is used by the regular expression engine to denote sequences of characters matched by marked sub-expressions. A match is a [begin, end) pair within the target range matched by the regular expression, but with additional observer functions to enhance code clarity.

Only the default constructor is publicly accessible. Instances of std::sub_match are normally constructed and populated as a part of a std::match_results container during the processing of one of the regex algorithms.

The member functions return defined default values unless the matched member is true.

std::sub_match inherits from std::pair<BidirIt,BidirIt>, although it cannot be treated as a std::pair object because member functions such as assignment will not work as expected.

Type requirements

-BidirIt must meet the requirements of LegacyBidirectionalIterator.

Specializations

Several specializations for common character sequence types are provided:

Defined in header `<regex>`
Type	Definition
`std::csub_match`	std::sub_match<constchar*>
`std::wcsub_match`	std::sub_match<constwchar_t*>
`std::ssub_match`	std::sub_match<std::string::const_iterator>
`std::wssub_match`	std::sub_match<std::wstring::const_iterator>

Nested types

Type	Definition
`iterator`	`BidirIt`
`value_type`	std::iterator_traits<BidirIt>::value_type
`difference_type`	std::iterator_traits<BidirIt>::difference_type
`string_type`	std::basic_string<value_type>

Data members

Member	Description
`matched`	whether this match was successful (public member object)

Inherited from std::pair

BidirIt first	start of the match sequence (public member object)
BidirIt second	one-past-the-end of the match sequence (public member object)

Member functions

(constructor)	constructs the match object (public member function)
Observers
length	returns the length of the match (if any) (public member function)
stroperator string_type	converts to the underlying string type (public member function)
compare	compares matched subsequence (if any) (public member function)
Modifiers
swap	swaps the contents (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)	compares a `sub_match` with another `sub_match`, a string, or a character (function template)
operator<<	outputs the matched character subsequence (function template)

Example

#include <cassert>
#include <iostream>
#include <regex>
#include <string>

int main()
{
    std::string sentence{"Friday the thirteenth."};
    const std::regex re{"([A-z]+) ([a-z]+) ([a-z]+)"};
    std::smatch words;
    std::regex_search(sentence, words, re);
    std::cout << std::boolalpha;
    for (const auto& m : words)
    {
        assert(m.matched);
        std::cout << "m: [" << m << "], m.length(): " << m.length() << ", "
                     "*m.first: '" << *m.first << "', "
                     "*m.second: '" << *m.second << "'\n";
    }
}

Output:

m: [Friday the thirteenth], m.length(): 21, *m.first: 'F', *m.second: '.'
m: [Friday], m.length(): 6, *m.first: 'F', *m.second: ' '
m: [the], m.length(): 3, *m.first: 't', *m.second: ' '
m: [thirteenth], m.length(): 10, *m.first: 't', *m.second: '.'