Starting loop at specific index of a std::string? - c++

Question

I wrote the following function:
std::regex r("");
for (std::sregex_iterator i = words_begin; i != words_end; ++i) {}
It starts looking for regex matches from the beginning of the given string (str) But how may I tell it to exclude everything before specific index?
For example I want it to delete with all of what comes after index number 4 (Not including it).
Note: I am calling this code from another function so I tried something like str + 4 in the string parameter but I got an error that it's not l-value.

Answer 1

If I understand your question correctly you can pass a parameter to the function with the position where you'd like to start the search, and use it to set the iterator:
void print_str(const std::string& str, int pos)
{
std::regex r("\\{[^}]*\\}");
auto words_begin =
std::sregex_iterator(str.begin() + pos, str.end(), r);
//...
}
int main()
{
std::string str = "somestring";
func_str(str, 4);
}
Or pass the iterators themselves, one to the position you'd like to start the search and one to the end of the string:
void func_str(std::string::iterator it_begin, std::string::iterator it_end)
{
std::regex r("\\{[^}]*\\}");
auto words_begin =
std::sregex_iterator(it_begin, it_end, r);
//...
}
int main()
{
std::string str = "somestring";
func_str(str.begin() + 4, str.end());
}
As #bruno correctly stated, you may use str.substr(4) not str + 4, as an argument instead of the original string, the downside of the method is that it will create unnecessary copies of the string to be searched, as #Marek also correctly pointed out, thus the options of passing a position or begin and end iterators is less expensive. The upside is that you would not have to change anything in the function.

Answer 2

I suggest checking the std::smatch#position() to determine if the match is to be taken or discarded:
#include <iostream>
#include<regex>
int main() {
std::regex r("\\{[^}]*\\}");
std::string str("{1}, {2} and {3}");
auto words_begin =
std::sregex_iterator(str.begin(), str.end(), r);
auto words_end = std::sregex_iterator();
for (std::sregex_iterator i = words_begin; i != words_end; ++i) {
std::smatch m = *i;
if (m.position() > 4) {
std::cout << m.str() << std::endl;
}
}
return 0;
}
See the C++ demo online. Adjust the if condition as you need.
Here, the first {1} match is discarded since its position was less or equal than 4.