Some characters cannot be included literally in string constants
("foo"
) or regexp constants (/foo/
).
Instead, they should be represented with escape sequences,
which are character sequences beginning with a backslash (`\').
One use of an escape sequence is to include a double-quote character in
a string constant. Because a plain double quote ends the string, you
must use `\"' to represent an actual double-quote character as a
part of the string. For example:
$ awk 'BEGIN { print "He said \"hi!\" to her." }' -| He said "hi!" to her.
The backslash character itself is another character that cannot be
included normally; you must write `\\' to put one backslash in the
string or regexp. Thus, the string whose contents are the two characters
`"' and `\' must be written "\"\\"
.
Backslash also represents unprintable characters such as TAB or newline. While there is nothing to stop you from entering most unprintable characters directly in a string constant or regexp constant, they may look ugly.
The following table lists all the escape sequences used in awk and what they represent. Unless noted otherwise, all these escape sequences apply to both string constants and regexp constants:
\\
\a
\b
\f
\n
\r
\t
\v
\
nnn\x
hh...
\/
\"
In gawk, a number of additional two-character sequences that begin with a backslash have special meaning in regexps. See GNU Regexp Operators.
In a regexp, a backslash before any character that is not in the previous list
and not listed in
GNU Regexp Operators,
means that the next character should be taken literally, even if it would
normally be a regexp operator. For example, /a\+b/
matches the three
characters `a+b'.
For complete portability, do not use a backslash before any character not shown in the previous list.
To summarize:
If you place a backslash in a string constant before something that is not one of the characters previously listed, POSIX awk purposely leaves what happens as undefined. There are two choices:
"a\qc"
is the same as "aqc"
.
(Because this is such an easy bug both to introduce and to miss,
gawk warns you about it.)
Consider `FS = "[ \t]+\|[ \t]+"' to use vertical bars
surrounded by whitespace as the field separator. There should be
two backslashes in the string `FS = "[ \t]+\\|[ \t]+"'.)
"a\qc"
is the same as typing
"a\\qc"
.
Suppose you use an octal or hexadecimal escape to represent a regexp metacharacter. (See Regexp Operators.) Does awk treat the character as a literal character or as a regexp operator?
Historically, such characters were taken literally.
(d.c.)
However, the POSIX standard indicates that they should be treated
as real metacharacters, which is what gawk does.
In compatibility mode (see Options),
gawk treats the characters represented by octal and hexadecimal
escape sequences literally when used in regexp constants. Thus,
/a\52b/
is equivalent to /a\*b/
.