Node:Escape Sequences, Next:Regexp Operators, Previous:Regexp Usage, Up:Regexp
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
0 and 7. For example, the code for the ASCII ESC
(escape) character is \033.
\xhh...
0-9, and either A-F
or a-f). Like the same construct
in ISO C, the escape sequence continues until the first nonhexadecimal
digit is seen. However, using more than two hexadecimal digits produces
undefined results. (The \x escape sequence is not allowed in
POSIX awk.)
\/
awk to keep processing the rest of the regexp.
\"
awk to keep processing the rest of the string.
In gawk, a number of additional two-character sequences that begin
with a backslash have special meaning in regexps.
See gawk-Specific Regexp Operators.
In a regexp, a backslash before any character that is not in the previous list
and not listed in
gawk-Specific 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:
awk reads your program.
gawk processes both regexp constants and dynamic regexps
(see Using Dynamic Regexps),
for the special operators listed in
gawk-Specific Regexp Operators.
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:
awk and gawk both do.
For example, "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]+".)
awk implementations do this.
In such implementations, typing "a\qc" is the same as typing
"a\\qc".
Suppose you use an octal or hexadecimal
escape to represent a regexp metacharacter.
(See Regular Expression 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 Command-Line 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/.