FULL PRODUCT VERSION :
java version " 1.7.0_09 "
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.5-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Mac OS X 10.7.5

A DESCRIPTION OF THE PROBLEM :
When matching against a regular expression that has a capturing group inside a possessive quantifier, the group sometimes shows captured input, even if it was not part of the final match.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
See source code below.

(1) compile pattern " ([abc]+?)(b)?+(d) "
(2) match against " abcd "
(3) check the value of matcher.group(2)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
For this regex, with Matcher " m " , I would expect m.group(2) to be null. More generally, since the three groups do not overlap, I'd expect " m.group(1) + m.group(2) + m.group(3) " to coincide with the input stream (if m.group(2) is not null).
ACTUAL -
Instead, m.group(2)= " b " , even though m.group(1)= " abc " and m.group(3)= " d " .

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.regex.*;

public class TestRegex {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile( " ([abc]+?)(b)?+(d) " );
        Matcher m = pattern.matcher( " abcd " );
        if(m.matches()) {
            System.out.println(m.group(0));
            System.out.println(m.group(1) + " | " + m.group(2) + " | " + m.group(3));
        } else {
            System.out.println( " does not match " );
        }
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
Don't use possessive quantifiers around capturing groups.