title: How to find all strings with length 5 that contain exactly one number and 4 letters
body: <p>I need regex to count strings with length of 5 that contain <strong>exactly</strong> one number (<code>0-9</code>) and 4 letters (<code>a-z</code>).  </p>
  
  <p>I succeed to find only letters with <code>^[a-z]{5}$</code> but I don't know can I tell it find strings that has exactly one number.</p>
  
  <p>For example:</p>
  
  <pre><code>jlwk6    -&gt; one number
  bjkgp
  5fm8s
  x975t
  k88q5
  zl796
  qm9hb    -&gt; one number
  h6gtf    -&gt; one number
  9rm9p
  jwzw2    -&gt; one number
  
  Total: 4
  </code></pre>