Converting std::string from const_iterator to char*?

You should do

int error=regexec(&regex,cppstring.c_str(),nmatch,pmatch,eflags);

The second argument of regexec() and the return value of std::string::c_str() are both (const char *), so that’s probably not the cause of your issue.

You should post the compiler error message and the part of the code that you suspect to be causing the problem.