std::match_results::length

From cppreference.com
difference_type length( size_type n = 0 ) const;
(since C++11)

Returns the length of the specified sub-match.

If n == 0, the length of the entire matched expression is returned.

If n > 0 && n < size(), the length of nth sub-match is returned.

if n >= size(), a length of the unmatched match is returned.

The call is equivalent to (*this)[n].length().

[edit] Parameters

n - integral number specifying which match to examine

[edit] Return value

The length of the specified match or sub-match.

[edit] See also

returns specified sub-match
(public member function)
returns the length of the match (if any)
(public member function of std::sub_match)