Emoji Unicode in regular expression with variable number of groups

Question

I am aware that this is a corner case, but I have come across a code that uses regular expression with variable number of groups

According to docs this is legal:

The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string "aba" against the expression (a(b)?)+, for example, leaves group two set to "b". All captured input is discarded at the beginning of each match.

However, when I try to use that with the unicode sign 'GRINNING FACE WITH SMILING EYES' (U+1F601) I get StringIndexOutOfBoundsException.

Is that expected according to the spec or a bug?

Here is the test code:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class TestEmoji {
    public static void main(String[] args)  {
        Pattern pattern = Pattern.compile("(A.)* EEE");

        testGroups(pattern, "ACAB EEE");
        testGroups(pattern,  "ABACA\uD83D\uDE01");

    }

    public static void testGroups(Pattern pattern, String s) {
        Matcher matcher = pattern.matcher(s);
        if (matcher.matches()) {
            System.out.println("matches");
            System.out.println(matcher.groupCount());
            for (int i = 1; i <= matcher.groupCount(); ++i) {
                System.out.println(matcher.group(i));
            }
        }
    }
}

and the exception:

matches
1
AB
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2
        at java.lang.String.charAt(String.java:658)
        at java.util.regex.Pattern$Slice.match(Pattern.java:3867)
        at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4382)
        at java.util.regex.Pattern$GroupCurly.match0(Pattern.java:4354)
        at java.util.regex.Pattern$GroupCurly.match(Pattern.java:4304)
        at java.util.regex.Matcher.match(Matcher.java:1221)
        at java.util.regex.Matcher.matches(Matcher.java:559)
        at TestEmoji.testGroups(TestEmoji.java:19)
        at TestEmoji.main(TestEmoji.java:12)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Answer 1

After some digging in Java Bugs database, I found it:

http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8007395

JDK-8007395 : StringIndexOutofBoundsException in Match.find() when input String contains surrogate UTF-16 characters