subcommand: Use POSIX compatible command regex.

subcommand: Use POSIX compatible command regex.

The POSIX way to ask for one or more matches of the preceeding atom
is {1,} while the plus character is a non-portable shortcut.

Note that since the closing brace of the regex bound is inside
parameter expansion, we need to escape it with a backslash while the
opening brace must not be escaped.