title: Single letter string representation by '?'
body: <p>I observed this in console and tried same in <code>irb</code>,</p>
  
  <pre><code>&gt; ?#'a'
   =&gt; "#a" 
  
  &gt; ?#
   =&gt; "#" 
  
  &gt; ??
   =&gt; "?" 
  
  &gt; ?..class
   =&gt; String
  
  &gt; self.method('?')
  # NameError (undefined method `?' for class `#&lt;Class:#&lt;Object:0x08ce9188&gt;&gt;')
  </code></pre>
  
  <p>Above works for single character while does not work for the case below </p>
  
  <pre><code>&gt; ?1
   =&gt; "1" 
  
  &gt; ?11
  Traceback (most recent call last):
          1: from /home/prs/.rvm/rubies/ruby-2.5.3/bin/irb:11:in `&lt;main&gt;'
  SyntaxError ((irb):33: syntax error, unexpected '?')
  </code></pre>
  
  <p>I could not find any documentation for above and could not get actual purpose of such implementation.</p>
  
  <p>Please help me for proper explanation and where this can be effectively used (appreciated if example provided).</p>