POSIX sucks when shells aren't implementing it correctly. POSIX says that PS1 expansion needs to support at least the bang ('!') expansion and the regular param

POSIX sucks when shells aren't implementing it correctly. POSIX says that PS1 expansion needs to support at least the bang ('!') expansion and the regular parameter expansion ('$' and '${'), but i found that several kinds of almquist shells don't support that even when they are explicitly POSIX-compliant, dash included. Bash in --posix mode does that perfectly.