New in version 3.9.

Capture group <n> matched by the last regular expression, for groups 0 through 9. Group 0 is the entire match. Groups 1 through 9 are the subexpressions captured by () syntax.

When a regular expression match is used, CMake fills in CMAKE_MATCH_<n> variables with the match contents. The CMAKE_MATCH_COUNT variable holds the number of match expressions when these are filled.