replace a unknown string between two known strings with sed

Question

I have a file with the following contents:

WORD1 WORD2 WORD3

How can I use sed to replace the string between WORD1 and WORD3 with foo

Answer 1

This might work for you:

sed 's/\S\+/foo/2' file

or perhaps:

sed 's/[^[:space:]][^[:space:]]*/foo/2' file

If WORD1 and WORD3 occur more than once:

echo "WORD1 WORD2 WORD3 BLA BLA WORD1 WORD4 WORD3" |
sed 's/WORD3/\n&/g;s/\(WORD1\)[^\n]*\n/\1 foo /g'
WORD1 foo WORD3 BLA BLA WORD1 foo WORD3

Answer 2

sed -i 's/WORD1.*WORD3/WORD1 foo WORD3/g' file.txt

or

sed -i 's/(WORD1).*(WORD3)/\1 foo \2/g' file.txt

You might need to escape round brackets, depends on your sed variant.

Answer 3

content of a sample file.txt

$ cat file.txt 
WORD1 WORD2 WORD3
WORD4 WORD5 WORD6
WORD7 WORD8 WORD9

(Correction by @DennisWilliamson in comment)
$ sed -e 's/\([^ ]\+\) \+\([^ ]\+\) \+\(.*\)/\1 foo \3/' file.txt

WORD1 foo WORD3
WORD4 foo WORD6
WORD7 foo WORD9

while awk is somehow simpler

$ awk -F' ' '{ print $1" foo "$3 }' file.txt

WORD1 foo WORD3
WORD4 foo WORD6
WORD7 foo WORD9