XML : Check if is present in xml file - JAVA

I'm working on a project with java jaxb - xml file. In order to validate an Xml I'm using all xsd validator and schema.validator etc etc, everything It's ok but I have a particular request: I must to check if an xml file contain this first line:

  <?xml version="1.0" encoding="UTF-8"?> .    

Now I have all data of xml file (String).

1. What can I do? So what is the regular expression to know if string exist ?
2. Can I use other validator instead String REGEX ? maybe some other jaxb validator?

Thanks all