CMAKE_MATCH_<n>

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.

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https://cmake.org/cmake/help/v3.15/variable/CMAKE_MATCH_n.html