Escapes are introduced with a dollar sign.  They are all
self-delimiting, and text that is not part of an escape is matched with

	  if (tolower(*np++) != tolower(*pp++)) fail();

The pattern is everything up to the first space or tab on each line.
Everything after the pattern has leading whitespace stripped and then
is used in reject message ("Not interested in mail from %s", sometimes
with an additional suffix if the mail comes through certain forwarding
hosts that I trust enough to walk their Received:s.)

All matching is greedy.  For example, if a $A is encountered, as many
letters as are available are consumed; if the match fails later, there
is no backtracking done.  Thus, for example, a pattern containing "$Ax"
can never match, even though from a regex perspective it could.  (This
is why the - flag exists for $O.)

In escape sequences, uppercase letters appear as-is; lowercase letters
are metasyntax, explained in the accompanying diescription.

Escape sequences:

$A
	One or more letters.

$D
	One or more digits.

$M
	One or more alphanumerics (letters and/or digits).

$L
	One or more letters/digits/dashes.  This is close to "one
	email-legal DNS label", and "label" is where the L comes from,
	but it's not quite.  (For these purposes, the difference
	doesn't matter.)

$Ofn
	n is a digit from 1 through 4; f is a # or - or nothing.  This
	matches a decimal number whose value equals one of the octets
	of the client's IP address.  The digit says which octet (1 is
	the most significant - first when written in the canonical
	dotted-quad notation - and 4 is the least/last).  Some number
	of digits are consumed, converted to a number, and the
	resulting number matched against the address octet.  Without
	flags, all available digits are consumed.  With the # flag, at
	most three; with the - flag, at most as many as the minimum
	needed to represent the octet in decimal.

$Xn
	n is a digit from 1 through 4.  This matches a two-digit
	hexadecimal number whose value equals one of the octets of the
	client's IP address.  n selects which octet, as for $O, and
	letters in the hex number may be either uppercase or lowercase.

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