The description which follows in the next section is an excerpt from the full specification of Exim, except that it lists only those expansion variables that are likely to be useful in filter files.
Expanded strings are copied verbatim except when a dollar or backslash character is encountered. A dollar specifies the start of a portion of the string which is interpreted and replaced as described below.
An uninterpreted dollar can be included in the string by putting a backslash in front of it -- if the string appears in quotes, two backslashes are required because the quotes themselves cause interpretation of backslashes when the string is read in. A backslash can be used to prevent any character being treated specially in an expansion, including itself.
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