Exim Filter Specification - More about string expansion

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## More about string expansion

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 from left to right 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 special character being treated specially in an expansion, including itself.

A backslash followed by one of the letters `n', `r', or `t' is recognized as an escape sequence for the character newline, carriage return, or tab, respectively. A backslash followed by up to three octal digits is recognized as an octal encoding for a single character, while a backslash followed by `x' and up to two hexadecimal digits is a hexadecimal encoding. A backslash followed by any other character causes that character to be added to the output string uninterpreted. These escape sequences are also recognized in quoted strings as they are read in; their interpretation in expansions as well is useful for unquoted strings and other cases such as looked-up strings that are then expanded.

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