Where do you see that's a regex? The doc link states:
`string(FIND <string> <substring> <output variable> [REVERSE])`
and
`FIND will return the position where the given substring was found in the supplied string. If the REVERSE flag was used, the command will search for the position of the last occurrence of the specified substring.`

There is no word on regex anywhere. Moreover, it doesn't work with `\\`!