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4.8 Literals

A literal is any sequence of characters enclosed in single or double quotes.

After tempfail and reject actions two special kinds of literals are recognized: three-digit numeric values represent RFC 2821 reply codes, and literals consisting of tree digit groups separated by dots represent an extended reply code as per RFC 1893/2034. For example:

510   # A reply code
5.7.1 # An extended reply code

Double-quoted strings

String literals enclosed in double quotation marks (double-quoted strings) are subject to backslash interpretation, macro expansion, variable interpretation and back reference interpretation.

Backslash interpretation is performed at compilation time. It consists in replacing the following escape sequences with the corresponding single characters:

SequenceReplaced with
\aAudible bell character (ASCII 7)
\bBackspace character (ASCII 8)
\fForm-feed character (ASCII 12)
\nNewline character (ASCII 10)
\rCarriage return character (ASCII 13)
\tHorizontal tabulation character (ASCII 9)
\vVertical tabulation character (ASCII 11)

Table 4.2: Backslash escapes

In addition, the sequence ‘\newline’ has the same effect as ‘\n’, for example:

"a string with\
 embedded newline"
"a string with\n embedded newline"

Any escape sequence of the form ‘\xhh’, where h denotes any hex digit is replaced with the character whose ASCII value is hh. For example:

"\x61nother" ⇒ "another"

Similarly, an escape sequence of the form ‘\0ooo’, where o is an octal digit, is replaced with the character whose ASCII value is ooo.

Macro expansion and variable interpretation occur at run-time. During these phases all Sendmail macros (see Sendmail Macros), mailfromd variables (see Variables), and constants (see Constants) referenced in the string are replaced by their actual values. For example, if the Sendmail macro f has the value ‘’ and the variable last_ip has the value ‘’, then the string11

"$f last connected from %last_ip;"

will be expanded to

" last connected from;"

A back reference is a sequence ‘\d’, where d is a decimal number. It refers to the dth parenthesized subexpression in the last matches statement12. Any back reference occurring within a double-quoted string is replaced by the value of the corresponding subexpression. See Special comparisons, for a detailed description of this process. Back reference interpretation is performed at run time.

Single-quoted strings

Any characters enclosed in single quotation marks are read unmodified.

The following examples contain pairs of equivalent strings:

"a string"
'a string'


Notice the last example. Single quotes are particularly useful in writing regular expressions (see Special comparisons).



Implementation note: actually, the references are not interpreted within the string, instead, each such string is split at compilation time into a series of concatenated atoms. Thus, our sample string will actually be compiled as:

$f . " last connected from " . last_ip . ";"

See Concatenation, for a description of this construct. You can easily see how various strings are interpreted by using --dump-tree option (see --dump-tree). In this case, it will produce:

        SYMBOL: f
        CONSTANT: " last connected from "
      VARIABLE last_ip (13)
    CONSTANT: ";"

The subexpressions are numbered by the positions of their opening parentheses, left to right.

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