code: function(inst) {
  			var allowedChars = (inst.options.multiSelect ? inst.options.multiSeparator :
  				(inst.options.rangeSelect ? inst.options.rangeSeparator : ''));
  			var literal = false;
  			var hasNum = false;
  			var dateFormat = inst.get('dateFormat');
  			for (var i = 0; i < dateFormat.length; i++) {
  				var ch = dateFormat.charAt(i);
  				if (literal) {
  					if (ch === "'" && dateFormat.charAt(i + 1) !== "'") {
  						literal = false;
  					}
  					else {
  						allowedChars += ch;
  					}
  				}
  				else {
  					switch (ch) {
  						case 'd': case 'm': case 'o': case 'w':
  							allowedChars += (hasNum ? '' : '0123456789'); hasNum = true; break;
  						case 'y': case '@': case '!':
  							allowedChars += (hasNum ? '' : '0123456789') + '-'; hasNum = true; break;
  						case 'J':
  							allowedChars += (hasNum ? '' : '0123456789') + '-.'; hasNum = true; break;
  						case 'D': case 'M': case 'Y':
  							return null; // Accept anything
  						case "'":
  							if (dateFormat.charAt(i + 1) === "'") {
  								allowedChars += "'";
  							}
  							else {
  								literal = true;
  							}
  							break;
  						default:
  							allowedChars += ch;
  					}
  				}
  			}
  			return allowedChars;
  		}
docs: Determine the set of characters allowed by the date format.
  @memberof CalendarsPicker
  @private
  @param inst {object} The current instance settings.
  @return {string} The set of allowed characters, or <code>null</code> if anything allowed.