Using a regex capture directly in expression in C++ - c++

Question

I'm trying to use a captured group directly in the regex. However, when I try to do this the program hangs indefinitely.
For example:
string input = "<Tag>blahblah</Tag>";
regex r1("<([a-zA-Z]+)>[a-z]+</\1>");
string result = regex_replace(result, regex, "");
If I add another slash to the capture "<([a-zA-Z]+)>[a-z]</\\1>", the program compiles but throws a "regex_error(regex_constants::error_backref)" exception.
Notes:
Compiler: Apple LLVM 5.1
I am using this as part of the process to clean junk from blocks of text. The document is not necessarily HTML/XML and desired text is not always within tags. So if possible, I would like to be able to do this with regular expressions, not a parser.

Answer 1

The backslash character in string literals is an escape character.
Either escape it "<([a-zA-Z]+)>[a-z]+</\\1>" or use a raw literal, R"(<([a-zA-Z]+)>[a-z]+</\1>)"
With that, your program works as you would expect:
#include <regex>
#include <iostream>
int main()
{
std::string input = "Hello<Tag>blahblah</Tag> World";
std::regex r1("<([a-zA-Z]+)>[a-z]+</\\1>");
std::string result = regex_replace(input, r1, "");
std::cout << "The result is '" << result << "'\n";
}
demo: http://coliru.stacked-crooked.com/a/ae20b09d46f975e9
The exception you're getting with \\1 suggests that your compiler is configured to use GNU libstdc++, where regex was not implemented. Look up how to set it up to use LLVM libc++ or use boost.regex.