c++ - 什么是正则表达式中的不匹配匹配？

Question

来自 https://en.cppreference.com/w/cpp/regex/match_results/str :

if n >= size(), a string representing the unmatched match is returned.

这是什么不匹配？

Answer 1

The call is equivalent to string_type((*this)[n]);

和operator[]提供更多信息:

if n >= size(), returns a reference to a std::sub_match representing an unmatched sub-expression (an empty subrange of the target sequence).