Re: param expansion with single-character special vars in the environment

[Dan Douglas]

...Also remember it isn't feasible to actually validate a "name" in a
script because a name can contain a subscript with a command
substitution that effectively requires parsing the full language.
(there are some tricks like with `set -nv` and reading its output to
shanghai the shell parser into doing your bidding, but that's not very
practical.). Before bash had namerefs, it could ignore aspects of
"invalid" names, like trailing characters after an array subscript,
which makes some valid ksh names at least get partially interpreted in
bash, like `a[foo].bar` or `a[foo][bar]`.