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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?
955
Regex Lookbehind is used as an assertion in Python regular expressions(re) to determine success or failure whether the pattern is behind i.e to the right of the parser's current position. They don't match anything. Hence, Regex Lookbehind and lookahead are termed as a zero-width assertion.
sed does not support lookaround assertions. For what it's worth, grep -P is also a nonstandard extension, though typically available on Linux (but not other platforms).
The lookbehind asserts that what immediately precedes the current position is a lowercase letter. And the lookahead asserts that what immediately follows the current position is an uppercase letter.
Lookarounds are zero width assertions. They check for a regex (towards right or left of the current position - based on ahead or behind), succeeds or fails when a match is found (based on if it is positive or negative) and discards the matched portion.
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
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