The function 'builtins.split' takes a POSIX extended regular expression and an arbitrary string. It returns a list of non-matching substring interleaved by lists of matched groups of the regular expression. ```nix with builtins; assert split "(a)b" "abc" == [ "" [ "a" ] "c" ]; assert split "([ac])" "abc" == [ "" [ "a" ] "b" [ "c" ] "" ]; assert split "(a)|(c)" "abc" == [ "" [ "a" null ] "b" [ null "c" ] "" ]; assert split "([[:upper:]]+)" " FOO " == [ " " [ "FOO" ] " " ]; ```
