Character Escaping

The character escaping facility allows escape sequences to occur within strings and quoted atoms, so that programmers can put non-printable characters in atoms and strings and still be able to see what they are doing. This facility can be switched on and off by the commands:

     | ?- prolog_flag(character_escapes, _, on).
     
     | ?- prolog_flag(character_escapes, _, off).
     

See ref-lps-ove, for a description of prolog_flag/3. Character escaping is off by default.

Strings or quoted atoms containing the following escape sequences can occur in terms obtained by read/[1,2], compile/1, and so on.

The 0' notation for the integer code of a character is also affected by character escaping.

With character escaping turned on, the only things that can occur in a string or quoted atom are the characters with ASCII codes 9 (horizontal tab), 32 (space), 33 through 126 (non-layout characters), or one of the following escape sequences:

The escape sequence:

Is converted to:

\b

backspace (ASCII 8)

\t

horizontal tab (ASCII 9)

\n

new line (ASCII 10)

\v

vertical tab (ASCII 11)

\f

form feed (ASCII 12)

\r

carriage return (ASCII 13)

\e

escape (ASCII 27)

\d

delete (ASCII 127)

\a

alarm = BEL (ASCII 7)

\xCD

the ASCII character with code CD (hexadecimal number)

\octal string

the ASCII character with code octal string base 8, where <octal string> is either 1, 2, or 3 octal digits

\^<control char>

the ASCII character whose code is the ASCII code of control char mod 32. \^? is another name for \d.

\layout char

no character, where layout char is a character with ASCII code =< 32 or ASCII code >= 127 (thus a newline or form-feed in a string or quoted atom can be ignored by immediately preceding it with a backslash)

\c

no character; also, all characters up to, but not including, the next non-layout character are ignored

\other

the character other, where other is any character not predefined here; thus \\ should be used to insert one backslash

It is an error if an escape sequence or ASCII character that is not defined above occurs in a string or quoted atom. For instance, an ordinary newline in an atom or string is regarded as an error when character escapes are on. This allows the syntax error of a missing closing quote to be caught much earlier, but it has the problem that some old programs will break (which is why character_escapses are off by default).

With character escaping turned on, the escape sequence \' represents the same character as the sequence '' within a quoted atom, namely one single quote. Similarly, with character escaping turned on, the escape sequence \" represents the same character as the sequence "" within a string, namely one double quote.

The escape sequence \c (c for continue) is useful when formatting a string for readability. For example, the atom (A), is equivalent to (B):

                '!Ruth \c (A)
                  Gehrig \c
                  Cobb \c
                  Williams!'
     

          '!Ruth Gehrig Cobb Williams!' (B)
     

The following sequence denotes the integer 9:

     0'\t