You need to first call Matcher#find
and then Matcher#group(1)
like this:
String str = "123123";
Pattern p = Pattern.compile("(\\d+)\\1");
Matcher m = p.matcher(str);
if (m.find())
System.out.println( m.group(1) ); // 123
PS: Your regex also needed some correction to use \\d+
instead of \\d
.