title: why grep promt "Invalid range end"?
body: <p>I have a file <code>a</code>:</p>
  
  <pre><code>$ cat a 
  abcd
  kaka
  </code></pre>
  
  <p>when using the command:</p>
  
  <pre><code>$ grep -e '[a-d]' a
  abcd
  kaka
  </code></pre>
  
  <p>It works well, but why those command is not right?</p>
  
  <pre><code>$ grep -e '[\x61-\x74]' a 
  grep: Invalid range end
  
  $ grep -e '[\u0061-\u0074]' a 
  grep: Invalid range end
  </code></pre>