Just reorder the cases so that that case is the last:
switch(ch){
case 'b':
//..
case 'c':
//..
case '_':
//...
case 'a':
//do something, condition does not match so go to default case
if (condition)
break;
//don't break in here, and don't allow fall through to other cases.
default:
//
break;
}