In almost all cases, regex is signified explicitly, usually by a menu selection that allows “matches” as an option, or for some actions like Search Variable it is always a regex.
But there are a few cases where a text field is almost always just a “contains” or “is” test (eg Button Press), and in some of those I allow regex if the entry starts with “^”.
I believe the only two cases of this are button matching and menu matching. It looks like button matching only applies to Press Button action, but menu matching probably applies to most menu matching facilities (like the menu condition).