title: regex to match entire words containing only certain characters
body: <p>I want to match entire words (or strings really) that containing only defined characters.</p>
  
  <p>For example if the letters are <code>d</code>, <code>o</code>, <code>g</code>:</p>
  
  <pre><code>dog = match
  god = match
  ogd = match
  dogs = no match (because the string also has an "s" which is not defined)
  gods = no match
  doog = match
  gd = match
  </code></pre>
  
  <p>In this sentence:</p>
  
  <pre><code>dog god ogd, dogs o
  </code></pre>
  
  <p>...I would expect to match on <code>dog</code>, <code>god</code>, and <code>o</code> (not <code>ogd,</code> because of the comma or <code>dogs</code> due to the <code>s</code>)</p>