I'm not sure what you mean. Commas in variables work fine:
//var %a = this,is,a,test | echo -a $regex(%a,/is/)
Maybe the problem is that you used commas directly in the <re> parameter. For example, this:
//var %a = this,is,a,test | echo -a $regex(%a,/this,is/)
will NOT work. As commas are the parameter separators in identifiers, they shouldn't be used directly inside parameters: they have to be escaped. In regex, you can use
\x2C, but generally you can use $chr(44). For example:
//echo -a $upper($+(this,$chr(44),is))