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

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< cpp‎ | regex‎ | match results

template< class OutputIt > OutputIter 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 > OutputIter 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

(none)

### Example

Run this code

    #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

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

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