title: sed - replacing text with colon
body: <p>I have this command:</p>
  
  <pre><code>sed -i  "s/a:b/b:c/g" file.txt
  </code></pre>
  
  <p>(in English: replace "a:b" with "b:c" in file.txt)</p>
  
  <p>This doesn't work because of the colons in the subsitution text.
  How should I re-write the command?</p>