When doing successive matches, track the input start and current search
start positions separately to prevent the `^` anchor from matching in
the middle of the string. Add policy CMP0186 to provide compatibility.
Issue: #26629Fixes: #16899
When `string(REGEX REPLACE)` or `string(REGEX MATCHALL)` loop
internally, they store their matches, but they do not clear the previous
match from an earlier iteration. This can leave the contents of
`CMAKE_MATCH_<N>` with bogus values for later matches in the string if
they have groups which earlier matched a non-empty string, but now match
an empty string.
Fixes#17079.