title: regex match single character plus exact string
body: <p>I have regex which match single letter like this <code>^[a-zA-Z]{1}$</code></p>
  <p>how to extend this to match not only single letter like a or b but also two additional strings like <code>0-9</code> and <code>All</code> ?</p>
  <p>So the regext should match</p>
  <pre><code>a yes
  b yes
  ab not
  1 not
  12 not
  1-9 yes
  something no
  all yes
  alli no
  </code></pre>