title: replace a unknown string between two known strings with sed
body: <p>I have a file with the following contents:</p>
  
  <p><code>WORD1 WORD2 WORD3</code></p>
  
  <p>How can I use sed to replace the string between WORD1 and WORD3 with <code>foo</code>, such that the contents of the file are changed to the following?:</p>
  
  <p><code>WORD1 foo WORD3</code></p>
  
  <p>I tried the following, but obviously I'm missing something because that does not produce the desired results:</p>
  
  <p><code>sed -i '' 's/WORD1.*WORD3/foo/g' file.txt</code></p>