mh-scan-msg-search-regexp

mh-scan-msg-search-regexp is a variable defined in `mh-scan.el'.
Its value is

"^[^0-9]*%d[^0-9]"

Documentation:
This regular expression matches a particular message.

It is a format string; use "%d" to represent the location of the
message number within the expression as in the default of
"^[^0-9]*%d[^0-9]".