String str = "abc,afda,fdadfae,fdaee,fdafda,efdaf";

Matcher m = Pattern.compile("(?<=^(?:[^,]+,){5})[^,]+(?=\\b|,)").matcher(str);
if(m.find())
{
    System.out.println(m.group(0));
}

Exception in thread "main" java.util.regex.PatternSyntaxException: Look-behind group does not have an obvious maximum length near index 17
(?<=^(?:[^,]+,){5})[^,]+(?=\b|,)
                 ^
    at java.util.regex.Pattern.error(Pattern.java:1713)
    at java.util.regex.Pattern.group0(Pattern.java:2488)
    at java.util.regex.Pattern.sequence(Pattern.java:1806)
    at java.util.regex.Pattern.expr(Pattern.java:1752)
    at java.util.regex.Pattern.compile(Pattern.java:1460)
    at java.util.regex.Pattern.<init>(Pattern.java:1133)
    at java.util.regex.Pattern.compile(Pattern.java:823)
    at com.rmouse.TestMagicCard.main(TestMagicCard.java:37)