32 Regular expressions library [re]

32.9 Class template match_results [re.results]

32.9.4 Size [re.results.size]

size_type size() const;
Returns: One plus the number of marked sub-expressions in the regular expression that was matched if *this represents the result of a successful match.
Otherwise returns 0.
[Note 1: 
The state of a match_results object can be modified only by passing that object to regex_match or regex_search.
Subclauses [re.alg.match] and [re.alg.search] specify the effects of those algorithms on their match_results arguments.
— end note]
size_type max_size() const;
Returns: The maximum number of sub_match elements that can be stored in *this.
[[nodiscard]] bool empty() const;
Returns: size() == 0.