It is mentioned in https://man.archlinux.org/man/PKGBUILD.5: 'The
variable is not allowed to contain colons, forward slashes, hyphens, or
whitespace.'
`_` is also an allowed character, and some software in the Arch Linux
AUR uses this naming convention.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>