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

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