Irule with Dynamic Regexp

Clarity would be helpful. Once you provide clear guidance on what you are trying to accomplish, I'd recommend you also take a look at scan instead of regex. Just some pointers to give you additional ideas:

% set opts [list "\[10\]" "\[43,11\]" "\[55,65,999\]"]
{[10]} {[43,11]} {[55,65,999]}
% foreach pattern $opts {
  set num [scan [string trim [string trim $pattern "\["] "\]"] %d,%d,%d x y z]
  switch $num {
  1 { puts Result: \n $x }
  2 { puts "$x, $y" }
  3 { puts "$x, $y, $z" }
  }
}
 Result:
 10
 43, 11
 55, 65, 999