void setup() {
std::string str = "The quick brown fox ABC3D97 jumps over the lazy wolf";
std::regex rgx {R"([A-Z]{3}\d{1}[A-Z]{1}\d{2})"};
std::smatch match;
if (!std::regex_search(str, match, rgx))
return; // fail, didn't match
auto output = match[0];
Serial.println(output);
}
It might be more efficient to start with simpler code. You should always look at the documentation and the examples: