mh-scan-msg-format-regexp

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

"%\\([0-9]*\\)(msg)"

Documentation:
This regular expression finds the message number width in a scan format.

Note that the message number must be placed in a parenthesized
expression as in the default of "%\\([0-9]*\\)(msg)". This
variable is only consulted if `mh-scan-format-file' is set to
"Use MH-E scan Format".