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Showing content from https://en.cppreference.com/w/cpp/language/../regex/regex_replace.html below:

std::regex_replace - cppreference.com

template< class OutputIt, class BidirIt, class Traits, class CharT,

          class STraits, class SAlloc >
OutputIt regex_replace( OutputIt out, BidirIt first, BidirIt last,
                        const std::basic_regex<CharT, Traits>& re,
                        const std::basic_string<CharT, STraits, SAlloc>& fmt,
                        std::regex_constants::match_flag_type flags =

                            std::regex_constants::match_default ); (1) (since C++11) (2) (since C++11) template< class Traits, class CharT,

          class STraits, class SAlloc, class FTraits, class FAlloc >
std::basic_string<CharT, STraits, SAlloc>
    regex_replace( const std::basic_string<CharT, STraits, SAlloc>& str,
                   const std::basic_regex<CharT, Traits>& re,
                   const std::basic_string<CharT, FTraits, FAlloc>& fmt,
                   std::regex_constants::match_flag_type flags =

                       std::regex_constants::match_default ); (3) (since C++11) template< class Traits, class CharT, class STraits, class SAlloc >

std::basic_string<CharT, STraits, SAlloc>
    regex_replace( const std::basic_string<CharT, STraits, SAlloc>& str,
                   const std::basic_regex<CharT, Traits>& re,
                   const CharT* fmt,
                   std::regex_constants::match_flag_type flags =

                       std::regex_constants::match_default ); (4) (since C++11) (5) (since C++11) (6) (since C++11)

regex_replace uses the regular expression re to perform substitution on the target character sequence:

1,2)

Copies characters in the range

[first, last)

to

out

, replacing any sequences that match

re

with characters formatted by

fmt

. Equivalent to:

using iter_type = std::regex_iterator<BidirIt, CharT, Traits>;
iter_type seq_begin(first, last, re, flags), seq_end;
 
using result_type = std::match_results<BidirIt>;
result_type m;
 
bool need_to_copy = (flags & std::regex_constants::format_no_copy) == 0;
bool format_all = (flags & std::regex_constants::format_first_only) != 0;
 
for (iter_type i = seq_begin; i != seq.end(); ++i)
{
    m = *i;
    if (need_to_copy)
        out = std::copy(m.prefix().first, m.prefix().second, out);
    if (format_all || i == seq_begin)
        out = /* replace-expr */
}
 
if (need_to_copy)
    out = m.ready()
              ? std::copy(m.suffix().first, m.suffix().second, out)
              : std::copy(first, last, out);
 
return out;

1) The expression /* replace-expr */ is m.format(out, fmt, flags).

2)

The expression

/* replace-expr */

is

m.format(out, fmt, fmt + std::char_traits<CharT>::length(fmt), flags)

.

[edit] Parameters first, last - the target character range str - the target std::string s - the target null-terminated C-style string re - the regular expression fmt - the regex replacement format string, exact syntax depends on the value of flags flags - flags used to determine how the match will be performed out - output iterator to store the result of the replacement [edit] Return value

As described above.

[edit] Exceptions

May throw std::regex_error to indicate an error condition.

[edit] Example 
#include <iostream>
#include <iterator>
#include <regex>
#include <string>
 
int main()
{
    std::string text = "Quick brown fox";
    std::regex vowel_re("a|e|i|o|u");
 
    // write the results to an output iterator
    std::regex_replace(std::ostreambuf_iterator<char>(std::cout),
                       text.begin(), text.end(), vowel_re, "*");
 
    // construct a string holding the results
    std::cout << '\n' << std::regex_replace(text, vowel_re, "[$&]") << '\n';
}

Output:

Q**ck br*wn f*x
Q[u][i]ck br[o]wn f[o]x
[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2213 C++11 out was not updated by the replacements out is updated [edit] See also attempts to match a regular expression to any part of a character sequence
(function template) [edit] options specific to matching
(typedef) [edit] replaces specified portion of a string
(public member function of std::basic_string<CharT,Traits,Allocator>) [edit]

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