As you determined, you need to use backslashes to escape the square brackets that indicate character classes in regular expressions.
The list_union() function takes two lists. If you need to do a union of more than two lists then you do
Code:
x = list_union(list1, list2, ',');
x = list_union(list3, x, ',')
etc, or
Code:
x = list_union(list1, list_union(list2, list3, ','), ',')