rmail-mime-charset-pattern

rmail-mime-charset-pattern is a variable defined in `rmail.el'.
Its value is

"^content-type:[ 	]*text/plain;\\(?:[ 	\n]*\\(?:format\\|delsp\\)=\"?[-a-z0-9]+\"?;\\)*[ 	\n]*charset=\"?\\([^ 	\n\";]+\\)\"?"

Documentation:
Regexp to match MIME-charset specification in a header of message.
The first parenthesized expression should match the MIME-charset name.