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I'm trying to use sed to find matching string on a line in a file on nth match. I want to use expression to mach num and "anything" after on that line and replace it with num x.

example file:

START num 1 num 1 num 1 num n END 
sed ':a;N;$!ba; s/num .*/num x/1' example

outputs:

START num x 
sed ':a;N;$!ba; s/num 1/num x/1' example

outputs:

START num x num 1 num 1 num n END

Im tyring to use the .* expression to match anything after num and replace it with num x but my result leaves the end of the file out. Replacing the .* expression with the actual full string on that line gives the desired result, but it must be possible that there is "anything" following characters on that line.

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    It's unclear what your expected output should look like. Please edit your question to add it. Commented Mar 27, 2020 at 15:35

2 Answers 2

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Your regex was not exactly right.

$ sed ':a;N;$!ba; s/\nnum[^\n]*/\nnum x/4' file
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sed is the best tool for doing s/old/new on individual lines, for anything else just use awk;

$ awk -v n=4 '/num/ && ((++c) == n){$0="num x"} {print}' file START num 1 num 1 num 1 num x END
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