title: What is the equivalent for epsilon in ANTLR BNF grammar notation?
body: <p>During taking advantage of ANTLR 3.3, I'm changing the current grammar to support inputs without parenthesis too. Here's the first version of my grammar :</p>
  
  <pre><code>grammar PropLogic;
  
          NOT : '!' ;
          OR  : '+' ;
          AND : '.' ;
          IMPLIES : '-&gt;' ;
          SYMBOLS : ('a'..'z') | '~' ;
          OP : '(' ;
          CP : ')' ;
  
      prog    : formula EOF ;
  
  
      formula : NOT formula
          | OP formula( AND formula CP | OR formula CP | IMPLIES formula CP)
          | SYMBOLS ;
  
  
      WHITESPACE : ( '\t' | ' ' | '\r' | '\n'| '\u000C' )+    { $channel = HIDDEN; } ;
  </code></pre>
  
  <p>Then I changed it this way to support the appropriate features :</p>
  
  <pre><code>grammar PropLogic;
  
      NOT : '!' ;
      OR  : '+' ;
      AND : '.' ;
      IMPLIES : '-&gt;' ;
      SYMBOL : ('a'..'z') | '~' ;
      OP : '(' ;
      CP : ')' ;
      EM : '' ;
  
  prog    : formula EOF ;
  
  
  formula : OP formula( AND formula CP | OR formula CP | IMPLIES formula CP)
      | ( NOT formula | SYMBOL )( AND formula | OR formula | IMPLIES formula | EM ) ;
  
  
  WHITESPACE : ( '\t' | ' ' | '\r' | '\n'| '\u000C' )+    { $channel = HIDDEN; } ;
  </code></pre>
  
  <p>But I've been faced with following error :</p>
  
  <pre><code>error&lt;100&gt;:  syntax error: invalid char literal: ''
  error&lt;100&gt;:  syntax error: invalid char literal: ''
  </code></pre>
  
  <p>Does anybody know that how can I overcome this error?</p>