ADDITIONAL SYSTEM INFORMATION :
Ubuntu
$ lsb_release -r
Release: 18.04
$ java -version
openjdk version "14" 2020-03-17
OpenJDK Runtime Environment (build 14+36-1461)
OpenJDK 64-Bit Server VM (build 14+36-1461, mixed mode, sharing)


A DESCRIPTION OF THE PROBLEM :
Regex doesn't match the pattern "X(@([^@]+)@)" correctly. There should be three groups generated and only the first two are. The third is missing.

REGRESSION : Last worked in version 14

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
0. Open a shell
1. Save the code below in a file called: REBug.java
2. Compile: javac REBug.java
3 Run the code in a shell: java REBug


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
m.groupCount()=2 <- should be 3
m.group(0)=X@b@
m.group(1)=@b@
m.group(2)=b <- missing (bug)
p: {a=X@b@}

ACTUAL -
m.groupCount()=2
m.group(0)=X@b@
m.group(1)=@b@
p: {a=X@b@}


---------- BEGIN SOURCE ----------
public class REBug {

    private static java.util.regex.Pattern p = java.util.regex.Pattern.compile(
        "X(@([^@]+)@)"
      // 1 2
    );

    public static String[] workaround(String pat) {
        String[] sa = new String[3];
        sa[0] = pat;
        sa[1] = pat.substring(1);
        sa[2] = pat.substring(2, pat.length() - 1);
        return sa;
    }

    public static void main(String[] args) throws Exception {
        String pat = "X@b@";
        java.util.regex.Matcher m = p.matcher(pat);
        //if (m.find()) { // same result
        //if (m.lookingAt()) { // same result
        if (m.matches()) {
System.err.println("# bug:");
System.err.println("m.groupCount()="+m.groupCount());
            for (int i = 0; i < m.groupCount(); ++i) {
System.err.println("m.group("+i+")="+m.group(i));
            }
        }

System.err.println("# workaround:");
        String[] sa = workaround(pat);
System.err.println("sa.length="+sa.length);
        for (int i = 0; i < sa.length; ++i) {
System.err.println("sa["+i+"]="+sa[i]);
        }
    }

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
See the workaround function provided (quick and dirty).

FREQUENCY : always