When you select Regexp in Trigger Manager the trigger match will actually be simple, and when you select Simple it will be Regexp. Also changing the mode removes special regexp characters from the string which is really annoying. Changing the mode should not affect the body string in any way.

Edit: Actually it doesn't seem to be this straightforward, but there is something weird with how those work. For example when you add a [.] to match '.' character the mode will change to simple when you save the trigger, but it still works as a regexp. Now when you change the mode back to regexp, the [.] will be removed from the body string.