private String checkCallbackName(String callbackName) throws IOException {
    if (null == callbackName) {
      return null;
    }
    if (callbackName.matches("[A-Za-z0-9_]+")) {
      return callbackName;
    }
    throw new IOException("'callback' must be alphanumeric");
  }
Verifies that the callback property, if provided, is purely alphanumeric. This prevents a
malicious callback name (that is javascript code) from being returned by the UI to an
unsuspecting user.
@param callbackName The callback name, can be null.
@return The callback name
@throws IOException If the name is disallowed.
hbase-http/src/main/java/org/apache/hadoop/hbase/http/jmx/JMXJsonServlet.java