Regex works only on first occurance? - c++

Question

Update: Kindly read my comment on jignatius's answer
I wrote the following code to find specific matches in a string using regex and to delete them and replace with another value, but it doesn't work as expected.
For example given the following input:
f={a,b}+{c,d}
I would expect it to delete both {a,b} and {c,d} but it only works on the first one, what is wrong with my code?
After Some checking I can see that the first loop is entered only once, but why?

Answer 1

There is a standard library function, std::regex_replace, in the header <regex> that does what to want to do: text replacement based on a regex. That will simplify things quite a lot for you instead of using a hand crafted loop.
You just need to supply the input string, the regex to match against, and the replacement string:
#include <iostream>
#include <regex>
#include <string>
int main()
{
std::regex reg(R"(\{[^}]*\})");
std::string mystring = "f={a,b}+{c,d}";
auto newstring = std::regex_replace(mystring, reg, "title");
std::cout << newstring; //f=title+title
}
Note: it's also easier to use a raw string literal with the format R"(literal)" to avoid using double backslashes to escape special characters in the regex.
Demo
In your comment you say that the replacement text can change. In that case, you will have to do a loop, not a straight forward regex replace.
You can use std::regex_iterator, a read-only forward iterator that will call std::regex_search() for you. You can use a string stream to build the new string:
#include <iostream>
#include <regex>
#include <string>
#include <sstream>
int main()
{
std::regex reg(R"(\{[^}]*\})");
std::string mystring = "f={a,b}+{c,d} + c";
std::vector<std::string> replacements = { "rep1", "rep2", "rep3" };
int i = 0;
auto start = std::sregex_iterator(mystring.begin(), mystring.end(), reg);
auto end = std::sregex_iterator{};
std::ostringstream ss;
for (std::sregex_iterator it = start; it != end; ++it)
{
std::smatch mat = *it;
ss << mat.prefix() << replacements[i++];
//If last match, stream suffix
if (std::next(it) == end)
{
ss << mat.suffix();
}
}
std::cout << ss.str(); //f=rep1+rep2 + c
}
Note that the prefix() method of the std::smatch object will give you the substring from the target string to the beginning of the match. Then you place your replacement text into the stream. Finally, you should use the suffix() method of the std::smatch object to stream any trailing text between the last match and the end of your target string.
Demo