C++ regex replace with a callback function - c++

Question

I have a map that stores id to value mapping, an input string can contain a bunch of ids. I need to replace those ids with their corresponding values. For example:
string = "I am in #1 city, it is now #2 time" // (#1 and #2 are ids here)
id_to_val_map = {1 => "New York", 2 => "summer"}
Desired output:
"I am in New York city, it is now summer time"
Is there a way I can have a callback function (that takes in the matched string and returns the string to be used as replacement) ? std::regex_replace doesn't seem to support that.
The alternative is to find all the matches, then compute their replacement values, and then perform the actual replacement. Which won't be that efficient.

Answer 1

You might do:
const std::map<int, std::string> m = {{1, "New York"}, {2, "summer"}};
std::string s = "I am in #1 city, it is now #2 time";
for (const auto& [id, value] : m) {
s = std::regex_replace(s, std::regex("#" + std::to_string(id)), value);
}
std::cout << s << std::endl;
Demo

Answer 2

A homegrown way is to use a while loop with regex_search() then
build the output string as you go.
This is essentially what regex_replace() does in a single pass.
No need to do a separate regex for each map item which has overhead of
reassignment on every item ( s=regex_replace() ) as well as covering the same
real estate with every pass.
Something like this regex
(?s)
( .*? ) # (1)
(?:
\#
( \d+ ) # (2)
| $
)
with this code
typedef std::string::const_iterator SITR;
typedef std::smatch X_smatch;
#define REGEX_SEARCH std::regex_search
std::regex _Rx = std::regex( "(?s)(.*?)(?:\\#(\\d+)|$)" );
SITR start = oldstr.begin();
SITR end = oldstr.end();
X_smatch m;
std::string newstr = "";
while ( REGEX_SEARCH( start, end, m, _Rx ) )
{
newstr.append( m[1].str() );
if ( m[2].matched ) {
{
// append the map keys value here, do error checking etc..
// std::string key = m[2].str();
int ndx = std::atoi( m[2].str() );
newstr.append( mymap[ ndx ] );
}
start = m[0].second;
}
// assign the old string with new string if need be
oldstr = newstr;