title: Nested quotes: RegEx to convert typographically correct cascading quotation
body: <p>While there are different examples on how to convert similar quotation marks into a typographically correct beginning and ending char by regular expressions, I am still searching for a way to professionally include the following requirement:</p>
  <p>A typographically <em>incorrect</em> way would be:</p>
  <blockquote>
  <p>I like the expression <strong>&quot;A traffic light shows &quot;orange&quot; during the switch from yellow to red!&quot;</strong></p>
  </blockquote>
  <p>Typographically <em>correct</em> a nested quote has to be shown in single quotes. That is similar in languages like English, German or French. In German (to clarify the difference in beginning and ending quotation marks) the desired result would be:</p>
  <blockquote>
  <p>Ich mag die Redewendung <strong>„Eine Ampel zeigt ‚orange‘ beim Umschalten von gelb zu rot!“</strong></p>
  </blockquote>
  <p>How can I get that result by using regular expressions in PHP?</p>