How to check for success in c++11 std::regex_replace? - c++

Question

I'd like to do the c++11 equivalent of a perl checked-replacement operation:
my $v = "foo.rat"
if ( $v =~ s/\.rat$/.csv/ )
{
...
}
I can do the replacement without trouble:
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string s{ "foo.rat" } ;
std::regex reg{ R"((.*)\.rat$)" } ;
s = std::regex_replace( s, reg, "$1.csv" ) ;
std::cout << s << std::endl ;
s = std::regex_replace( "foo.noo", reg, "$1.csv" ) ;
std::cout << s << std::endl ;
return 0 ;
}
This gives:
foo.csv
foo.noo
Notice that the replace operation on the non-matching expression doesn't throw an error (which is what I expected).
Looking at the regex_replace documentation, it's not obvious to me how to check for the success of the replace operation. I could do a string compare, but that seems backwards?

Answer 1

Try to find match with std::regex_match or std::regex_search, check if something is matched, then replace found portion of string using std::string::replace. That shouldn't lead to performance loss.

Answer 2

Just to add to the accepted answer that it can also be done with a std::regex_iterator. This may be handy when multiple replacements may took place.
Iterator std::regex_iterator repeatedly calls std::regex_search() until all matches are found. If the position of the iterator at the beginning and the position at the end are the same, no match was found.
Function bool regex_replace(std::string &str, const std::string &re, const std::string& replacement) implements this behaviour:
#include <iostream>
#include <regex>
bool regex_replace(std::string &str, const std::string &re, const std::string& replacement) {
std::regex regexp(re);
//Search regex
std::sregex_iterator begin = std::sregex_iterator(str.begin(), str.end(), regexp);
std::sregex_iterator end = std::sregex_iterator();
//replace using iterator
for (std::sregex_iterator i = begin; i != end; ++i)
str.replace(i->position(), i->length(), replacement);
//returns true if at least one match was found and replaced
return (begin != end);
}
This function operates in place. At the end str have the replacements. Only if any replacement was made, the function returns true.
Following code shows how to use it to make multiple replacements and detect if any was made:
int main(int argc, char** argv) {
std::string rgx("[0-9]");
std::string str("0a1b2c3d4e5");
std::string replacement("?");
bool found = regex_replace(str, rgx, replacement);
std::cout << "Found any: " << (found ? "true" : "false") << std::endl;
std::cout << "string: " << str << std::endl;
return 0;
}
The code substitutes every digit for the quotation mark '?':
Found any: true
string: ?a?b?c?d?e?

Answer 3

use std::regex_constants::format_no_copy flag to change the behavior of regex_replace(). look the code below.
the return string will now be empty if match failed.
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string s{ "foo.rat" } ;
std::regex reg{ R"((.*)\.rat$)" } ;
auto rxMatchFlag = std::regex_constants::format_no_copy; //<---use this to modify the behavior of regex_replace when matching failed.
s = std::regex_replace( s, reg, "$1.csv", rxMatchFlag) ;
if(!s.empty()) std::cout << s << std::endl ;
else std::cout << "failed match" << std::endl;
s = std::regex_replace( "foo.noo", reg, "$1.csv", rxMatchFlag) ;
if(!s.empty()) std::cout << s << std::endl ;
else std::cout << "failed match" << std::endl;
return 0 ;
}
for the other flags, look them here

Answer 4

I don't believe there's any direct way to find out whether any replacements were made.
(Don't confuse this with "success / not success", which is not quite the same thing.)