simple java regex with backreference does not work

Question

I have trouble to replace a string with a backreference of a regular expression: Nothing is replaced and i always end up with my input.
My code:

String input="A12.3 bla bla my input";
input = StringUtils.replacePattern(
            input, "^([A-Z]\\d{2}\\.\\d)",
            "$1");
System.out.println(input);

The main problem is that I can not change the java code but only the input, regex and group reference.

Do you have any suggestions about any other regex pattern that matches my needs or what is going wrong?

---

StringUtils is of Apache Commons Lang

Answer 1

You are replacing same matched pattern by itself. Probably you meant:

String input="A12.3 bla bla my input";
input = StringUtils.replacePattern(
            input, "^([A-Z]\\d{2}\\.\\d).*$", "$1");
//                                      ^^^
System.out.println(input);

.*$ will match the input till end.