mh-scan-msg-number-regexp

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

"^ *\\([0-9]+\\)"

Documentation:
This regular expression extracts the message number.

It must match from the beginning of the line. Note that the
message number must be placed in a parenthesized expression as in
the default of "^ *\\([0-9]+\\)".