mh-scan-cur-msg-number-regexp

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

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

Documentation:
This regular expression matches the current message.

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 and current message
marker "+" within the parenthesis since it looks better to
highlight these items as well. The highlighting is done with the
face `mh-folder-cur-msg-number'. This regular expression should
be correct as it is needed by non-fontification functions. See
also `mh-note-cur'.