you need to learn at least the basics of Tcl before asking for help with a script; from your question I gather that you don't know a thing about string manipulation (or else you wouldn't be asking how to filter data which starts with a particular letter), so please get back to your Tcl book or/and tcl.tk and learn about [string]
well yeah hehehe... if you need to parse for just a single or couple of data elements, using tDOM/XPath doesn't make much sense and would be an overkill; however, if you have to implement complex parsing logic and there's good chance for that webpage to change, it would be much easier to go with tDOM and simply adjust XPath as necessary