How to replace first n no. of occurrence of a string using sed and/or awk?

Question

I got a file alphabet where a has multiple occurrence in a line.

$ cat alphabet
a b c d e f g 
h i j k a a l
m n a p q r a
s t u v w a x
y z a k l q z

where

$ cat alphabet | grep -o a | wc -l
7

Now how can I replace only first 3 occurrence of a with Z so that my file looks like as follows

Z b c d e f g 
h i j k Z Z l
m n a p q r a
s t u v w a x
y z a k l q z

Answer 1

awk '{
    for (i=1; i<=NF; i++) 
        if ($i == "a" && n < 3) {
            n++
            $i = "Z"
        }
    print
}' alphabet

Or, "one-liner"-ed

awk '{for (i=1;i<=NF;i++) if ($i=="a" && n++<3) $i="Z"; print}' alphabet

Answer 2

Perl to the rescue:

perl -pe '$c++ while $c < 3 && s/a/Z/' alphabet

Answer 3

Here, the sed way

sed -E ':a;N;$!ba;s#a#Z#;s#a#Z#;s#a#Z#' alphabet

Since sed normaly works on lines, any command to sed will act only on 1 line at a time. To be able to replace only the first 3 occurances we need to first make the whole file a single selection on which we will do our 3 replacements. Otherwise we will do 3 replacements on each line.

• :a creates a label
• N appends next line into patterns space
• $! skips last newline
• ba branches to label a

We have now selected the whole file and will be acting on that space instead on one line at a time, do 3 replacements of "a" with "Z".

The above command will only work on GNU sed, more general but a bit uglier version that should work on non-GNU sed:

sed -e ':a' -e 'N' -e '$!ba' -e 's#a#Z#' -e 's#a#Z#' -e 's#a#Z#' alphabet

EDIT: As suggested in the comments, adding version which uses g command to first replace all occurrences of 'a' with 'Z' and then replace all occurrences of 'Z' after 3th with 'a' again, which effectively leads to replacing only the first 3 occurrences of 'a'. This way you can change the last number to reflect the number of substitutions you need.

sed -e ':a;N;$!ba;s#a#Z#g;s#Z#a#g4' alphabet

Answer 4

The awk solution that has been posted assumes that all the occurrences of a are separate words.  While this is true of the example data, it is not specified as being true of the real data.  The following awk solution is more in the spirit of the perl solution that has been posted:

awk '{ while (changes < 3  &&  sub("a", "Z") > 0) changes++; print }' alphabet

This replaces (substitutes) occurrences of a with Z until the changes counter reaches 3.  Of course, to actually change the file, you will need to do something like

awk '{while (c < 3 && sub("a","Z")>0) c++; print}' alphabet > t && cp t alphabet && rm t

where t is a temporary file.