String text = "asdf 2013-05-12 asdf";
String regex = ".+?([0-9]{4}s?-s?[0-9]{2}s?-s?[0-9]{2}).+";
Matcher matcher = Pattern.compile(regex).matcher(text);
if (matcher.matches()) {
String matchedGroup = matcher.group(1);
System.out.println(matchedGroup);
}