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Built-in Functions for String Manipulation

The functions in this section look at or change the text of one or more strings. Optional parameters are enclosed in square brackets ("[" and "]").

index(in, find)
This searches the string in for the first occurrence of the string find, and returns the position in characters where that occurrence begins in the string in. For example:
$ awk 'BEGIN { print index("peanut", "an") }'
-| 3
If find is not found, index returns zero. (Remember that string indices in awk start at one.)
This gives you the number of characters in string. If string is a number, the length of the digit string representing that number is returned. For example, length("abcde") is five. By contrast, length(15 * 35) works out to three. How? Well, 15 * 35 = 525, and 525 is then converted to the string "525", which has three characters. If no argument is supplied, length returns the length of $0. In older versions of awk, you could call the length function without any parentheses. Doing so is marked as "deprecated" in the POSIX standard. This means that while you can do this in your programs, it is a feature that can eventually be removed from a future version of the standard. Therefore, for maximal portability of your awk programs, you should always supply the parentheses.
match(string, regexp)
The match function searches the string, string, for the longest, leftmost substring matched by the regular expression, regexp. It returns the character position, or index, of where that substring begins (one, if it starts at the beginning of string). If no match is found, it returns zero. The match function sets the built-in variable RSTART to the index. It also sets the built-in variable RLENGTH to the length in characters of the matched substring. If no match is found, RSTART is set to zero, and RLENGTH to -1. For example:
awk '{
       if ($1 == "FIND")
         regex = $2
       else {
         where = match($0, regex)
         if (where != 0)
           print "Match of", regex, "found at", \
                     where, "in", $0
This program looks for lines that match the regular expression stored in the variable regex. This regular expression can be changed. If the first word on a line is `FIND', regex is changed to be the second word on that line. Therefore, given:
FIND ru+n
My program runs
but not very quickly
FIND Melvin
This line is property of Reality Engineering Co.
Melvin was here.
awk prints:
Match of ru+n found at 12 in My program runs
Match of Melvin found at 1 in Melvin was here.
split(string, array [, fieldsep])
This divides string into pieces separated by fieldsep, and stores the pieces in array. The first piece is stored in array[1], the second piece in array[2], and so forth. The string value of the third argument, fieldsep, is a regexp describing where to split string (much as FS can be a regexp describing where to split input records). If the fieldsep is omitted, the value of FS is used. split returns the number of elements created. The split function splits strings into pieces in a manner similar to the way input lines are split into fields. For example:
split("cul-de-sac", a, "-")
splits the string `cul-de-sac' into three fields using `-' as the separator. It sets the contents of the array a as follows:
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
The value returned by this call to split is three. As with input field-splitting, when the value of fieldsep is " ", leading and trailing whitespace is ignored, and the elements are separated by runs of whitespace. Also as with input field-splitting, if fieldsep is the null string, each individual character in the string is split into its own array element. (This is a gawk-specific extension.) Recent implementations of awk, including gawk, allow the third argument to be a regexp constant (/abc/), as well as a string (d.c.). The POSIX standard allows this as well. Before splitting the string, split deletes any previously existing elements in the array array (d.c.).
sprintf(format, expression1,...)
This returns (without printing) the string that printf would have printed out with the same arguments (see section Using printf Statements for Fancier Printing). For example:
sprintf("pi = %.2f (approx.)", 22/7)
returns the string "pi = 3.14 (approx.)".
sub(regexp, replacement [, target])
The sub function alters the value of target. It searches this value, which is treated as a string, for the leftmost longest substring matched by the regular expression, regexp, extending this match as far as possible. Then the entire string is changed by replacing the matched text with replacement. The modified string becomes the new value of target. This function is peculiar because target is not simply used to compute a value, and not just any expression will do: it must be a variable, field or array element, so that sub can store a modified value there. If this argument is omitted, then the default is to use and alter $0. For example:
str = "water, water, everywhere"
sub(/at/, "ith", str)
sets str to "wither, water, everywhere", by replacing the leftmost, longest occurrence of `at' with `ith'. The sub function returns the number of substitutions made (either one or zero). If the special character `&' appears in replacement, it stands for the precise substring that was matched by regexp. (If the regexp can match more than one string, then this precise substring may vary.) For example:
awk '{ sub(/candidate/, "& and his wife"); print }'
changes the first occurrence of `candidate' to `candidate and his wife' on each input line. Here is another example:
awk 'BEGIN {
        str = "daabaaa"
        sub(/a*/, "c&c", str)
        print str
-| dcaacbaaa
This shows how `&' can represent a non-constant string, and also illustrates the "leftmost, longest" rule in regexp matching (see section How Much Text Matches?). The effect of this special character (`&') can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write `\\&' in a string constant to include a literal `&' in the replacement. For example, here is how to replace the first `|' on each line with an `&':
awk '{ sub(/\|/, "\\&"); print }'
Note: As mentioned above, the third argument to sub must be a variable, field or array reference. Some versions of awk allow the third argument to be an expression which is not an lvalue. In such a case, sub would still search for the pattern and return zero or one, but the result of the substitution (if any) would be thrown away because there is no place to put it. Such versions of awk accept expressions like this:
sub(/USA/, "United States", "the USA and Canada")
For historical compatibility, gawk will accept erroneous code, such as in the above example. However, using any other non-changeable object as the third parameter will cause a fatal error, and your program will not run.
gsub(regexp, replacement [, target])
This is similar to the sub function, except gsub replaces all of the longest, leftmost, non-overlapping matching substrings it can find. The `g' in gsub stands for "global," which means replace everywhere. For example:
awk '{ gsub(/Britain/, "United Kingdom"); print }'
replaces all occurrences of the string `Britain' with `United Kingdom' for all input records. The gsub function returns the number of substitutions made. If the variable to be searched and altered, target, is omitted, then the entire input record, $0, is used. As in sub, the characters `&' and `\' are special, and the third argument must be an lvalue.
gensub(regexp, replacement, how [, target])
gensub is a general substitution function. Like sub and gsub, it searches the target string target for matches of the regular expression regexp. Unlike sub and gsub, the modified string is returned as the result of the function, and the original target string is not changed. If how is a string beginning with `g' or `G', then it replaces all matches of regexp with replacement. Otherwise, how is a number indicating which match of regexp to replace. If no target is supplied, $0 is used instead. gensub provides an additional feature that is not available in sub or gsub: the ability to specify components of a regexp in the replacement text. This is done by using parentheses in the regexp to mark the components, and then specifying `\n' in the replacement text, where n is a digit from one to nine. For example:
$ gawk '
>      a = "abc def"
>      b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
>      print b
> }'
-| def abc
As described above for sub, you must type two backslashes in order to get one into the string. In the replacement text, the sequence `\0' represents the entire matched text, as does the character `&'. This example shows how you can use the third argument to control which match of the regexp should be changed.
$ echo a b c a b c |
> gawk '{ print gensub(/a/, "AA", 2) }'
-| a b c AA b c
In this case, $0 is used as the default target string. gensub returns the new string as its result, which is passed directly to print for printing. If the how argument is a string that does not begin with `g' or `G', or if it is a number that is less than zero, only one substitution is performed. gensub is a gawk extension; it is not available in compatibility mode (see section Command Line Options).
substr(string, start [, length])
This returns a length-character-long substring of string, starting at character number start. The first character of a string is character number one. For example, substr("washington", 5, 3) returns "ing". If length is not present, this function returns the whole suffix of string that begins at character number start. For example, substr("washington", 5) returns "ington". The whole suffix is also returned if length is greater than the number of characters remaining in the string, counting from character number start. Note: The string returned by substr cannot be assigned to. Thus, it is a mistake to attempt to change a portion of a string, like this:
string = "abcdef"
# try to get "abCDEf", won't work
substr(string, 3, 3) = "CDE"
or to use substr as the third agument of sub or gsub:
gsub(/xyz/, "pdq", substr($0, 5, 20))  # WRONG
This returns a copy of string, with each upper-case character in the string replaced with its corresponding lower-case character. Non-alphabetic characters are left unchanged. For example, tolower("MiXeD cAsE 123") returns "mixed case 123".
This returns a copy of string, with each lower-case character in the string replaced with its corresponding upper-case character. Non-alphabetic characters are left unchanged. For example, toupper("MiXeD cAsE 123") returns "MIXED CASE 123".

More About `\' and `&' with sub, gsub and gensub

When using sub, gsub or gensub, and trying to get literal backslashes and ampersands into the replacement text, you need to remember that there are several levels of escape processing going on.

First, there is the lexical level, which is when awk reads your program, and builds an internal copy of your program that can be executed.

Then there is the run-time level, when awk actually scans the replacement string to determine what to generate.

At both levels, awk looks for a defined set of characters that can come after a backslash. At the lexical level, it looks for the escape sequences listed in section Escape Sequences. Thus, for every `\' that awk will process at the run-time level, you type two `\'s at the lexical level. When a character that is not valid for an escape sequence follows the `\', Unix awk and gawk both simply remove the initial `\', and put the following character into the string. Thus, for example, "a\qb" is treated as "aqb".

At the run-time level, the various functions handle sequences of `\' and `&' differently. The situation is (sadly) somewhat complex.

Historically, the sub and gsub functions treated the two character sequence `\&' specially; this sequence was replaced in the generated text with a single `&'. Any other `\' within the replacement string that did not precede an `&' was passed through unchanged. To illustrate with a table:

This table shows both the lexical level processing, where an odd number of backslashes becomes an even number at the run time level, and the run-time processing done by sub. (For the sake of simplicity, the rest of the tables below only show the case of even numbers of `\'s entered at the lexical level.)

The problem with the historical approach is that there is no way to get a literal `\' followed by the matched text.

The 1992 POSIX standard attempted to fix this problem. The standard says that sub and gsub look for either a `\' or an `&' after the `\'. If either one follows a `\', that character is output literally. The interpretation of `\' and `&' then becomes like this:

This would appear to solve the problem. Unfortunately, the phrasing of the standard is unusual. It says, in effect, that `\' turns off the special meaning of any following character, but that for anything other than `\' and `&', such special meaning is undefined. This wording leads to two problems.

  1. Backslashes must now be doubled in the replacement string, breaking historical awk programs.
  2. To make sure that an awk program is portable, every character in the replacement string must be preceded with a backslash.(11)

The POSIX standard is under revision.(12) Because of the above problems, proposed text for the revised standard reverts to rules that correspond more closely to the original existing practice. The proposed rules have special cases that make it possible to produce a `\' preceding the matched text.

In a nutshell, at the run-time level, there are now three special sequences of characters, `\\\&', `\\&' and `\&', whereas historically, there was only one. However, as in the historical case, any `\' that is not part of one of these three sequences is not special, and appears in the output literally.

gawk 3.0 follows these proposed POSIX rules for sub and gsub. Whether these proposed rules will actually become codified into the standard is unknown at this point. Subsequent gawk releases will track the standard and implement whatever the final version specifies; this book will be updated as well.

The rules for gensub are considerably simpler. At the run-time level, whenever gawk sees a `\', if the following character is a digit, then the text that matched the corresponding parenthesized subexpression is placed in the generated output. Otherwise, no matter what the character after the `\' is, that character will appear in the generated text, and the `\' will not.

Because of the complexity of the lexical and run-time level processing, and the special cases for sub and gsub, we recommend the use of gawk and gensub for when you have to do substitutions.

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