Google forms - Multiple choice OR checkbox - other regular expression

This is not a RegEx building question. Is there an option to add RegEx validation to the "other" option in either the multiple choice or the checkbox question type? In other words, users would be able to select a set option OR they would be allowed to add their own option ("other"). This own option should meet the regex. I cannot find such an option via the form design tool but was wondering if this is possible via the script. Thanks