Open the form to create a global regular expression. There, create a subexpression "aaa,bbb,ccc" of type "Any character string included".
Now, switch to "Test" tab and test the expression with string "aaa". The expected results is "TRUE", whereas the frontend gives us "FALSE". Only when "aaa,bbb,ccc" is a substring of the string to test does it seem to give us a "TRUE" result.