I read an article about a regular expression to detect base64 but when I try it in "yara python" it gives an error of "unterminated regular expression"
the regular expression is: 

(?:[A-Za-z0-9+/]{4}){2,}(?:[A-Za-z0-9+/]{2}[AEIMQUYcgkosw048]=|[A-Za-z0-9+/][AQgw]==)

could anyone throw a suggestion please?
thanks
I would suggest escaping / character in the [A-Za-z0-9+/] block, because while unescaped it defines regular expression start/end.
regular Expression to detect base64