title: How to match any combination of letters using regex?
body: <p>How can I match letters a,b,c once in any combination and varying length like this:</p>
  
  <p>The expression should match these cases:</p>
  
  <pre><code>abc
  bc
  a
  b
  bca
  </code></pre>
  
  <p>but should not match these ones:</p>
  
  <pre><code>abz
  aab
  cc
  x
  </code></pre>