gnus: Regular Expressions Header Matching

 
 9.17.6.6 Regular Expressions Header Matching
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  -- Variable: spam-use-regex-headers
 
      This option is disabled by default.  You can let Gnus check the
      message headers against lists of regular expressions when you set
      this option.  The variables ‘spam-regex-headers-spam’ and
      ‘spam-regex-headers-ham’ hold the list of regular expressions.
      Gnus will check against the message headers to determine if the
      message is spam or ham, respectively.
 
  -- Variable: spam-regex-headers-spam
 
      The list of regular expressions that, when matched in the headers
      of the message, positively identify it as spam.
 
  -- Variable: spam-regex-headers-ham
 
      The list of regular expressions that, when matched in the headers
      of the message, positively identify it as ham.
 
    Regular expression header checks are done only on incoming mail.
 There is no specific spam or ham processor for regular expressions.