double square bracket in 'case' in 'configure.ac' i[[3456]]86

I am trying to understand the following code snippet. host_cpu='i386' case "$host_cpu" in i[]86) echo "host_cpu=i386" ;; x86_64) echo "host_cpu=x86_64" ;; *) echo "AC_MSG_ERROR([unsupported CPU type]) " ;; esac I added the variable host_cpu='i386' myself to test the code and it switched to the third case of echo "AC_MSG_ERROR([unsupported CPU type]) ". If I change the double bracket in i[]86) to a single bracket as in i86), it switches to the first option yielding i386. This seems correct to me. I understand that [ and [[ are the test options. The test condition doesn't seem to apply here as the switch cases are expecting a character to be outputted. So I'm assuming for bash to pick it up as a test condition, it needs to be separated by a space as in [ a < b ] or [[ a << b ]]. Because there are no spaces in these case statement, it'll be treated as a regular expression. Is this correct? So my question is why was the double square bracket was used here by the code writer? It didn't work when I tried to run the code, so what was their intention. Note: The code was taken from a configure.ac in the GRUB source code. Also the $host_cpu and host_cpu=i386 line seem unnecessary, can you explain why the writer would have done this: AC_CANONICAL_HOST case "$host_cpu" in i[]86) host_cpu=i386 ;; x86_64) host_cpu=x86_64 ;; *) AC_MSG_ERROR([unsupported CPU type]) ;; esac AC_SUBST(host_cpu) AC_SUBST(host_vendor) I'm thinking of using AC_SUBST($host_cpu). Why wouldn't you do it this way?