Please add support for underscores in triplet names

[cjdb]

Describe the bug
My toolchain files are named according to the toolchain triplet that they work with (e.g. config/cmake/toolchains/linux/x86_64-unknown-linux-gnu.cmake). I also want my triplets to have the same name as their chainload for consistency (e.g. config/cmake/toolchains/vcpkg/x86_64-unknown-linux-gnu.cmake).

Underscores were permitted until either a very late 2024 release, or sometime in 2025.

Environment

$ cmake --version
cmake version 4.0.1
$ vcpkg/vcpkg --version
vcpkg package management program version 2025-04-07-b8b513ba8778c918cff49c3e837aae5999d5d2aa

To Reproduce
Steps to reproduce the behavior:

1. Create a triplet with an underscore in its name (e.g. x86_64-unknown-linux-gnu.cmake).
2. Configure with -DVCPKG_TARGET_TRIPLET=x86_64-unknown-linux-gnu.

Expected behavior
Underscores are an accepted character.

Failure logs

error: Invalid triplet name. Triplet names are all lowercase alphanumeric+hyphens.
  on expression: x86_64-unknown-linux-llvm-asan
                    ^
Built-in Triplets:
<snip>

[StarGate-One]

See rationale for change with 41268.

[cjdb]

See rationale for change with 41268.

Thanks, I was struggling to find something, but I was looking at the source instead of GitHub. Seems this is a feature request, not a regression.

I am curious as to why underscores aren't permitted from a design perspective, given that they're used in multiple toolchain triplets. What does it cost to add this as a supported feature?

[StarGate-One]

Do not know why just that it was broken and the fix no longer allows the underscore.