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std::match_results<BidirIt,Alloc>::format

template< class OutputIt >
OutputIt format( OutputIt out,
                 const char_type* fmt_first, const char_type* fmt_last,
                 std::regex_constants::match_flag_type flags =
                     std::regex_constants::format_default ) const;
 (1) (since C++11) 
template< class OutputIt, class ST, class SA >
OutputIt format( OutputIt out,
                 const basic_string<char_type,ST,SA>& fmt,
                 std::regex_constants::match_flag_type flags =
                     std::regex_constants::format_default ) const;
 (2) (since C++11) 
template< class ST, class SA >
std::basic_string<char_type,ST,SA> 
    format( const std::basic_string<char_type,ST,SA>& fmt,
            std::regex_constants::match_flag_type flags =
                std::regex_constants::format_default ) const;
 (3) (since C++11) 
string_type format( const char_type* fmt_s,
                    std::regex_constants::match_flag_type flags =
                        std::regex_constants::format_default ) const;
 (4) (since C++11)

format outputs a format string, replacing any format specifiers or escape sequences in that string with match data from *this.

1) The format character sequence is defined by the range [fmt_first, fmt_last). The resulting character sequence is copied to out.
2) The format character sequence is defined by the characters in fmt. The resulting character sequence is copied to out.
3-4) The format character sequence is defined by the characters in fmt and fmt_s respectively. The resulting character sequence is copied to a newly constructed std::basic_string, which is returned.

The flags bitmask determines which format specifiers and escape sequences are recognized.

The behavior of format is undefined if ready() != true.

Parameters

fmt_begin, fmt_end - pointers to a range of characters defining the format character sequence
fmt - std::basic_string defining the format character sequence
fmt_s - pointer to a null-terminated character string defining the format character sequence
out - iterator that the resulting character sequence is copied to
flags - std::regex_constants::match_flag_type bitmask specifying which format specifiers and escape sequences are recognized
Type requirements
-OutputIt must meet the requirements of LegacyOutputIterator.

Return value

1-2) out
3-4) The newly constructed string containing resulting character sequence.

Exceptions

May throw implementation-defined exceptions.

Example

#include <iostream>
#include <string>
#include <regex>
 
int main()
{
    std::string s = "for a good time, call 867-5309";
    std::regex phone_regex("\\d{3}-\\d{4}");
    std::smatch phone_match;
 
    if (std::regex_search(s, phone_match, phone_regex)) {
        std::string fmt_s = phone_match.format(
            "$`"    // $` means characters before the match
            "[$&]"  // $& means the matched characters
            "$'");  // $' means characters following the match
        std::cout << fmt_s << '\n';
    }   
}

Output:

for a good time, call [867-5309]

See also

(C++11)
 replaces occurrences of a regular expression with formatted replacement text
(function template)
(C++11)
 options specific to matching
(typedef)

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https://en.cppreference.com/w/cpp/regex/match_results/format