In addition to the traditional true or false conditions you can specify
on a recipe, you can use a weighted scoring technique to decide if
a certain recipe matches or not. When weighted scoring is used in a
recipe, then the final score for that recipe must be positive for it
to match.
A certain condition can contribute to the score if you allocate it
a `weight'
(w)
and an `exponent'
(x).
You do this by preceding the condition (on the same line) with:
w^x
Whereas both
w
and
x
are real numbers between -2147483647.0 and 2147483647.0 inclusive.
Weighted regular expression conditions
The first time the regular expression is found, it will add
w
to the score. The second time it is found,
w*x
will be added. The third time it is found,
w*x*x
will be added. The fourth time
w*x*x*x
will be added. And so forth.
This can be described by the following concise formula:
n
n k-1 x - 1
w * Sum x = w * -------
k=1 x - 1
It represents the total added score for this condition if
n
matches are found.
Note that the following case distinctions can be made:
x=0
Only the first match will contribute w to the score. Any subsequent
matches are ignored.
x=1
Every match will contribute the same w to the score. The score grows
linearly with the number of matches found.
0<x<1
Every match will contribute less to the score than the previous one.
The score will asymptotically approach a certain value (see the
NOTES
section below).
1<x
Every match will contribute more to the score than the previous one.
The score will grow exponentially.
x<0
Can be utilised to favour odd or even number of matches.
If the regular expression is negated (i.e., matches if it isn't found),
then
n
obviously can either be zero or one.
Weighted program conditions
If the program returns an exitcode of EXIT_SUCCESS (=0), then the total
added score will be
w.
If it returns any other exitcode (indicating failure), the total added
score will be
x.
If the exitcode of the program is negated, then, the exitcode will
be considered as if it were a virtual number of matches. Calculation
of the added score then proceeds as if it had been a normal regular
expression with
n=`exitcode'
matches.
Weighted length conditions
If the length of the actual mail is
M
then:
* w^x > L
will generate an additional score of:
x
/ M \
w * | --- |
\ L /
And:
* w^x < L
will generate an additional score of:
x
/ L \
w * | --- |
\ M /
In both cases, if L=M, this will add w to the score. In the former case
however, larger mails will be favoured, in the latter case, smaller
mails will be favoured. Although x can be varied to fine-tune the
steepness of the function, typical usage sets x=1.
MISCELLANEOUS
You can query the final score of all the conditions on a recipe from the
environment variable
$=.
This variable is set
every
time just after procmail has parsed all conditions on a recipe (even if the
recipe is not being executed).
EXAMPLES
The following recipe will ditch all mails having more than 150 lines in the
body.
The first condition contains an empty regular expression which, because
it always matches, is used to give our score a negative offset.
The second condition then matches every line in the mail, and consumes
up the previous negative offset we gave (one point per line). In the end,
the score will only be positive if the mail contained more than 150 lines.
:0 Bh
* -150^0
* 1^1 ^.*$
/dev/null
Suppose you have a priority folder which you always read first. The next
recipe picks out the priority mail and files them in this special folder.
The first condition is a regular one, i.e., it doesn't contribute to the
score, but simply has to be satisfied. The other conditions describe things
like: john and claire usually have something important to say, meetings
are usually important, replies are favoured a bit, mails about Elvis
(this is merely an example :-) are favoured (the more he is mentioned, the
more the mail is favoured, but the maximum extra score due to Elvis will
be 4000, no matter how often he is mentioned), lots of quoted lines are
disliked, smileys are appreciated (the score for those will reach a maximum
of 3500), those three people usually don't send
interesting mails, the mails should preferably be small (e.g., 2000 bytes long
mails will score -100, 4000 bytes long mails do -800).
As you see, if some of the uninteresting people send mail, then the mail
still has a chance of landing in the priority folder, e.g., if it is about
a meeting, or if it contains at least two smileys.
If you are subscribed to a mailinglist, and just would like to read
the quality mails, then the following recipes could do the trick.
First we make sure that the mail is coming from the mailinglist.
Then we check if it is from certain persons of whom we value
the opinion, or about a subject we absolutely want to know everything
about. If it is, file it. Otherwise, check if the ratio of quoted lines
to original lines is at most 1:2. If it exceeds that, ditch the mail.
Everything that survived the previous test, is filed.
For further examples you should look in the
procmailex(5)
man page.
CAVEATS
Because this speeds up the search by an order of magnitude,
the procmail internal egrep will always search for the leftmost
shortest
match, unless it is determining what to assign to
MATCH,
in which case it searches the leftmost
longest
match.
E.g. for the leftmost
shortest
match, by itself, the regular expression:
.*
will always match a zero length string at the same spot.
.+
will always match one character (except newlines of course).
