You can just use the pipe on its own:
"string1|string2"
for example:
String s = "string1, string2, string3";
System.out.println(s.replaceAll("string1|string2", "blah"));
Output:
blah, blah, string3
The main reason to use parentheses is to limit the scope of the alternatives:
String s = "string1, string2, string3";
System.out.println(s.replaceAll("string(1|2)", "blah"));
has the same output. but if you just do this:
String s = "string1, string2, string3";
System.out.println(s.replaceAll("string1|2", "blah"));
you get:
blah, stringblah, string3
because you've said "string1" or "2".
If you don't want to capture that part of the expression use ?::
String s = "string1, string2, string3";
System.out.println(s.replaceAll("string(?:1|2)", "blah"));