Rule to delete emails with utf-8 encoded subjects

I'm trying to create a mail flow rule which will delete emails whose subject header is utf-8 encoded. What this means is that the email subject would start with =?utf-8?

However, I can't seem to create a rule that does this.  I Tried:

Subject matches these text patterns ... '=\?utf-8\?'

also tried

A message header matches ...  'Subject' header matches '=\?utf-8\?'

However, both doesn't work.  Any ideas or help would be much appreciated. 

Thanks!