title: PCRE conditional subpatterns: (R) as the condition
body: <p>This is from the PHP <a href="http://php.net/manual/en/regexp.reference.conditional.php" rel="nofollow">manual</a> regarding PCRE conditional subpatterns:</p>
  
  <blockquote>
    <p>The two possible forms of conditional subpattern are:<br/>
    <code>(?(condition)yes-pattern)</code><br/>
    <code>(?(condition)yes-pattern|no-pattern)</code><br/></p>
  </blockquote>
  
  <p>That's OK as long as the condition is a digit or an assertion. But I don't quite understand the following </p>
  
  <blockquote>
    <p>If the condition is the string (R), it is satisfied  if a recursive
    call to the pattern or subpattern has been made.  At "top level", the
    condition is false. (...) If the condition is not a sequence of digits
    or (R), it must be an assertion.</p>
  </blockquote>
  
  <p>I would be grateful if someone could explain <strong>on an example</strong> what (R) is in conditional subpattern and how to use it. Thanks in advance.</p>