title: Regex - unknown place of character
body: <p>I have a string like as <code>abcd</code> but the place of <code>a</code> is unknown. The string can be <code>bacd</code> or <code>bcad</code> or <code>bcda</code>.</p>
  
  <p>I tried</p>
  
  <pre><code>a{0,1}ba{0,1}ca{0,1}da{0,1}
  </code></pre>
  
  <p>but this pattern matches <code>bcd</code> too.</p>
  
  <p>I need exactly one occurrence of <code>a</code> with unknown place.
  <code>a</code> is a symbol. Iwant to use sth like as <code>b[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]</code> Instead of <code>a</code>.</p>