title: grep with regexp: whitespace doesn't match unless I add an assertion
body: <p>GNU grep 2.5.4 on bash 4.1.5(1) on Ubuntu 10.04</p>
  
  <p>This matches</p>
  
  <pre><code>$ echo "this is a     line" | grep 'a[[:space:]]\+line'
  this is a     line
  </code></pre>
  
  <p>But this doesn't</p>
  
  <pre><code>$ echo "this is a     line" | grep 'a\s\+line'
  </code></pre>
  
  <p>But this matches too</p>
  
  <pre><code>$ echo "this is a     line" | grep 'a\s\+\bline'
  this is a     line
  </code></pre>
  
  <p>I don't understand why #2 does not match (whereas # 1 does) and #3 also shows a match. Whats the difference here?</p>