CMake negative lookahead (RegExp support)

Is there any way to use (?!...) (the negative-lookahead pattern), and somehow check result in condition?

Or, is there any cross-platform way of running RegEx externally?
(as my reasearch shows, CMake may never support RegExp fully!!)

Example:

I am using CMake version 3.20.5 (current latest stable).

Example of what I tried is below:

if (CMAKE_SYSTEM_PROCESSOR MATCHES "^arm(?!(64))")
    message(FATAL_ERROR "Got this far!!")
endif()

And for now, I worked it around like:

if (CMAKE_SYSTEM_PROCESSOR MATCHES "^arm" AND NOT CMAKE_SYSTEM_PROCESSOR MATCHES "^arm64")
   # ...
endif()

Error log:

/my-project/CMakeLists.txt:60: error: given arguments: "CMAKE_SYSTEM_PROCESSOR" "MATCHES" "^arm(?!(64))" Regular expression "^arm(?!(64))" cannot compile

My research:

I found the report "CMake Regex does not support lookahead regular expressions" on issue-tracker (which is clone of original report).

With original explanation:

Yeah, that isn't supported.
Adding support would likely mean rewriting the regex engine
(which is old, global variable-driven C code).

It seems, CMake developers are damn lazy (not fixing since 2016-02-09),
and maybe they don't even intend to ever support RegExp fully (because issue-tracker shows "Assignees = None", "Milestone = None", "Labels = None").