I need to easily identify a text document as an RSS feed of any type
(0.9, 1.0, 1.1, 2.0 are all ok), an Atom feed of any type or neither. 
I need to do so using the first 100 characters or so of the potential
RSS/Atom feed.  I am not asking for an RSS/Atom parser.

Answer needs to include:
RSS feeds start with any of the following text:  string_a, string_b, string_c, etc
Atom feeds start with any of the following text: string_d, string_e, string_f, etc

I am fairly competent with Regex and string manipulation, so I can can
code this method/script myself.  I just need to know what to look for!