title: awk regular expression
body: <p>I am learning regular expressions in AWK. In AWK <code>[^ ]</code> <strong>matches any character not inside</strong>. Now I need to extract those '<strong>a</strong>' which are not part of any word like.</p>
  
  <p>If my input file is </p>
  
  <pre><code>a
  aa
  a+a
  .a.a
  bac
  </code></pre>
  
  <p>The output will be </p>
  
  <pre><code>a+a
  .a.a
  </code></pre>
  
  <p>Because <em>a+a</em> and <em>.a.a</em> are not any words so these lines are printed.
  So I did something like this </p>
  
  <pre><code>awk '/[^[a-z]]a[^[a-z]]/ {print} input_file'
  </code></pre>
  
  <p>But this is not working. Please Help!</p>