mh-scan-refiled-msg-regexp

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

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

Documentation:
This regular expression matches refiled messages.

It must match from the beginning of the line. Note that the
default setting of `mh-folder-font-lock-keywords' expects this
expression to contain at least one parenthesized expression which
matches the message number as in the default of

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

This expression includes the leading space within the parenthesis
since it looks better to highlight it as well. The highlighting
is done with the face `mh-folder-refiled'. This regular
expression should be correct as it is needed by non-fontification
functions. See also `mh-note-refiled'.