Has anyone confirmed this behaviour directly against the runtimes/languages? Newlines at the end of a string are certainly something that could get lost in transit inside an online service involving multiple runtimes.
Beyond what's in the OP, that includes RE2, Hyperscan, D's std.regex, ICU, Perl, Python's third party `regex` package, and `regress`.
I've also run that locally against "go1.22.1 darwin/arm64", "go1.21.5 windows/amd64", and "go1.21.0 linux/amd64" with the same result.
> The ^ and $ language elements indicate the beginning and end of the input string. The end of the input string can be a trailing newline \n character.
In what way could newlines at the end of a string "could get lost in transit"?
There are plenty of other ways, too; bugs happen.