$ a='[a]' ksh -c 'case $a in $a) echo match; esac'
match
$ a='\a' ksh -c 'case $a in $a) echo match; esac'
$ a='[a]' b='[\a]' ksh -c 'case $a in $b) echo match; esac'
match

Some historical shells would fall back to doing a byte to byte comparison with each pattern if the pattern matching rules did not produce a match. That behavior is not allowed by this standard because it allows user input to bypass input validations like:

    case $1 in
      [0123456789]) : OK;;
      *) echo >&2 not a decimal digit; exit 1;;
    esac