title: Regular expressions fixed prefix
body: <p>I have the regex string and I want to classify, is this regex has fixed prefix or no.</p>
  
  <p>For example:</p>
  
  <p><code>abcdef.*g[0abc]{0,5}hi</code> has prefix <code>abcdef</code></p>
  
  <p><code>]1234vac.*12345</code> has prefix <code>]1234vac</code></p>
  
  <p><code>(abc)+123</code> has prefix <code>abc</code></p>
  
  <p>but</p>
  
  <p><code>[A-z]+12345</code> doesn't has fixed prefix (it starts from unknown number of symbols from set A-z)</p>
  
  <p>Am I truly understand that this problem will not be solved in a general form?</p>