public static String replaceVariables(ExpressionContext ctx,
    String text) 
{
      String value = text;
      try
      {
         REMatch[] matches = variableRE.getAllMatches(text);
         if( matches.length  > 0 )
         {
            StringBuffer tmp = new StringBuffer();
            for(int m = 0; m <  matches.length; m ++)
            {
               String prefix = matches[m].toString(1);
               String name = matches[m].toString(2);
               String suffix = matches[m].toString(3);
               QName varName = new QName(name);
               XObject var = ctx.getVariableOrParam(varName);
               tmp.append(prefix);
               if( var != null )
                  tmp.append(var.toString());
               tmp.append(suffix);
            }
            value = tmp.toString();
         }
      }
      catch(TransformerException e)
      {
         e.printStackTrace();
      }
      return value;
}

This function replaces all occurrences of variable references ${...} with the corresponding XSL variable. If no such variable is defined the variable is replaced with an empty string.

 public static  void setVariableRE(RE variableRE) 
{
      Util.variableRE = variableRE;
}