Why does this regex throw an exception?

I am trying to use std::regex_replace in C++11 (Visual Studio 2013) but the regex i am trying to create is throwing an exception:

Microsoft C++ exception: std::regex_error at memory location 0x0030ED34

Why is this the case? This is my definition:

std::string regexStr = R"(\([A - Za - z] | [0 - 9])[0 - 9]{2})";

std::regex rg(regexStr); <-- This is where the exception thrown

line = std::regex_replace(line, rg, this->protyp->getUTF8Character("$&"));

What i want to do: Find all matches inside a string which are of the following format:

"\X99" OR "\999" where X = A-Z or a-z and 9 = 0-9.

I also tried to use the boost regex library, but it also throws an exeception.

(Another question: Can i use the backreference as i am doing in the last line? I want to replace dynamically according to the match)

Thanks for any help