code: public String ensureValidExpression(String dataSource, String attrName, String errorId)
              throws JspException
      {
          try {
              boolean isExpr = isExpression(dataSource);
  
              // @perf: if the isExpr call fails, this is an error condition, and the containsExpression
              // call cost is irrelevant
              if (!isExpr && containsExpression(dataSource)) {
                  String s = Bundle.getString(errorId, new Object[]{dataSource});
                  _tag.registerTagError(s, null);
                  return null;
              }
  
              if (!isExpr) {
                  String s = Bundle.getString(errorId, new Object[]{dataSource});
                  _tag.registerTagError(s, null);
                  return null;
              }
          }
          catch (Exception e) {
              // pass throw JspExceptions
              if (e instanceof JspException)
                  throw (JspException) e;
  
              String s = Bundle.getString(errorId, new Object[]{dataSource});
              _tag.registerTagError(s, e);
              return null;
          }
          return dataSource;
      }
docs: Ensure that the passed in data source is a valid expression.
  @param dataSource
  @param attrName
  @param errorId
  @return String
  @throws javax.servlet.jsp.JspException