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