C++ 正则表达式捕获组混淆

Question

I'm implementing the nand2tetris Assembler in C++ (I'm pretty new to C++), and I'm having a lot of trouble parsing a C-instruction using regex. Mainly I really don't understand the return value of regex_search and how to use it.

Setting aside the various permutations of a C instruction, the current example I'm having trouble with is D=DM . The result should have dest = "D"; comp = "DM" dest = "D"; comp = "DM" .

With the current code below, the regex appears to find the results correctly (confirmed by regex101.com), but, not really correctly, or something, or I don't know how to get to it. See the debugger screenshot. matches[n].second (which appears to contain the correct comp value) is not a string but an iterator.

Note that the 3rd capture group is correctly empty for this example.

auto regex_str = regex("([AMD]{1,3}=)?([01\-AMD!|+&><]{1,3})?(;[A-Z]{3})?");
regex_search(assemblyCode, matches, regex_str);
string dest = matches[1]; // this automatically casts some object (submatch) into a string?
string comp = matches[2]; 
string jump = matches[3];

I will note, though, that D=D+M works, but not D=DM !

Answer 1

gcc warns about unknows escape sequence \- Demo .

You have to escape \ ,

std::regex("([AMD]{1,3}=)?([01\\-AMD!|+&><]{1,3})?(;[A-Z]{3})?");

or use raw string

std::regex(R"(([AMD]{1,3}=)?([01\-AMD!|+&><]{1,3})?(;[A-Z]{3})?)");

Demo