Obeying commands conditionally

Most of the power of filtering comes from the ability to test conditions and obey different commands depending on the outcome. The `if' command is used to specify conditional execution, and its general form is

if    <condition>
then  <commands>
elif  <condition>
then  <commands>
else  <commands>
endif

There may be any number of `elif' and `then' sections (including none) and the `else' section is also optional. Any number of commands, including nested `if' commands, may appear in any of the <commands> sections.

Conditions can be combined by using the words `and' and `or', and round brackets (parentheses) can be used to specify how several conditions are to combine. Without brackets, `and' is more binding than `or'. For example,

if
  $h_subject: contains "Make money" or
  $h_precedence: is "junk" or
  ($h_sender: matches ^\\d{8}@ and not personal) or
  $message_body contains "this is spam"
then
  seen finish
endif

A condition can be preceded by `not' to negate it, and there are also some negative forms of condition that are more English-like.

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