string - What is an alternative for lookbehind with C++ RegEx?

Question

I am using the following pattern:

(?<=<)(?<!>).*?q.*?(?!<)(?=>)

It uses positive and negative lookahead and lookbehind to match the literal q that is enclosed in matching brackets.

std::regex does not support lookbehind. So what would be a good alternative?

Answer 1

Note that (?<=<)(?<!>) is equal to (?<=<) (since a < is required immediately to the left of the current location, there cannot be any >) and (?!<)(?=>) is equal to (?=>) (same logic applies here, as > must be immediately to the right, there won't be any <). The first .*? will not match the shortest substring possible, it will literally find its way to the first q that is followed with any 0+ chars up to the first >. So, the pattern is hardly working for you even in the lookbehind-supporting engine.

I'd rather use <([^<>q]*q[^<>]*)> regex with a capturing group and literal consuming < and > symbols at the start/end of the expression:

std::regex r("<([^<>q]*q[^<>]*)>");
std::string s = "<adqsdq<><abc>5<abq>6<qaz> <hjfffffffk>";
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
                         i != std::sregex_iterator();
                         ++i)
{
    std::cout << (*i).str(1)  << srd::endl;
}

See the C++ demo

Output: abq and qaz