FULL PRODUCT VERSION :
java version "1.8.0_73"
Java(TM) SE Runtime Environment (build 1.8.0_73-b02)
Java HotSpot(TM) Client VM (build 25.73-b02, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Windows 10

EXTRA RELEVANT SYSTEM CONFIGURATION :
In Matcher.java line 854 is written:
                    // The first number is always a group
                    refNum = (int)nextChar - '0';
                    if ((refNum < 0)||(refNum > 9))
                        throw new IllegalArgumentException(
                            "Illegal group reference");

That the first character after $ is always a number is not correct.

A DESCRIPTION OF THE PROBLEM :
That the first character after $ is always a number is not correct.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
public static void main(String[] args) {
final String k = "1234";
System.out.println(k.replaceAll("23", "$&"));
}

ACTUAL -
Exception in thread "main" java.lang.IllegalArgumentException: Illegal group reference
at java.util.regex.Matcher.appendReplacement(Matcher.java:857)
at java.util.regex.Matcher.replaceAll(Matcher.java:955)
at java.lang.String.replaceAll(String.java:2210)
at overlay.StartupTest.main(StartupTest.java:52)


ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.IllegalArgumentException: Illegal group reference
at java.util.regex.Matcher.appendReplacement(Matcher.java:857)
at java.util.regex.Matcher.replaceAll(Matcher.java:955)
at java.lang.String.replaceAll(String.java:2210)
at overlay.StartupTest.main(StartupTest.java:52)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public static void main(String[] args) {
final String k = "1234";
System.out.println(k.replaceAll("23", "$&"));
}
---------- END SOURCE ----------