C++ Regular expressions regex_iterator Example
Example
When processing of captures has to be done iteratively a regex_iterator is a good choice. Dereferencing a regex_iterator returns a match_result. This is great for conditional captures or captures which have interdependence. Let's say that we want to tokenize some C++ code. Given:
enum TOKENS {
NUMBER,
ADDITION,
SUBTRACTION,
MULTIPLICATION,
DIVISION,
EQUALITY,
OPEN_PARENTHESIS,
CLOSE_PARENTHESIS
};
We can tokenize this string: const auto input = "42/2 + -8\t=\n(2 + 2) * 2 * 2 -3"s with a regex_iterator like this:
vector<TOKENS> tokens;
const regex re{ "\\s*(\\(?)\\s*(-?\\s*\\d+)\\s*(\\)?)\\s*(?:(\\+)|(-)|(\\*)|(/)|(=))" };
for_each(sregex_iterator(cbegin(input), cend(input), re), sregex_iterator(), [&](const auto& i) {
if(i[1].length() > 0) {
tokens.push_back(OPEN_PARENTHESIS);
}
tokens.push_back(i[2].str().front() == '-' ? NEGATIVE_NUMBER : NON_NEGATIVE_NUMBER);
if(i[3].length() > 0) {
tokens.push_back(CLOSE_PARENTHESIS);
}
auto it = next(cbegin(i), 4);
for(int result = ADDITION; it != cend(i); ++result, ++it) {
if (it->length() > 0U) {
tokens.push_back(static_cast<TOKENS>(result));
break;
}
}
});
match_results<string::const_reverse_iterator> sm;
if(regex_search(crbegin(input), crend(input), sm, regex{ tokens.back() == SUBTRACTION ? "^\\s*\\d+\\s*-\\s*(-?)" : "^\\s*\\d+\\s*(-?)" })) {
tokens.push_back(sm[1].length() == 0 ? NON_NEGATIVE_NUMBER : NEGATIVE_NUMBER);
}
A notable gotcha with regex iterators is that the regex argument must be an L-value, an R-value will not work: Visual Studio regex_iterator Bug?
