If you go the regular expression way...
Code:
/(<iframe)(.*?)(<\/iframe>)/ig
...will match "iframe" tag content (more or less), and...
...will match the "src" attribute value within the result of the previous match.
Even if it is for the Perl programming language and not Java, a good, relatively compact regular expressions tutorial can be found here:
http://perldoc.perl.org/perlretut.html
A good book on the subject is "Mastering Regular Expressions":
http://shop.oreilly.com/product/9780596528126.do