FULL PRODUCT VERSION :
java version "1.8.0_73"
Java(TM) SE Runtime Environment (build 1.8.0_73-b02)
Java HotSpot(TM) 64-Bit Server VM (build 25.73-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
WIndows 10, online Java, etc.

A DESCRIPTION OF THE PROBLEM :
After matching the regular expression ab(?:a(c))*ac over the string abacacac,
querying the start position of the matched group (c) returns 7, which is not possible since the c at position c is matched by the last c of the regular expression. And (?: ...) is consuming, so there is no look-ahead.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Paste the following code into an online java IDE such as: https://www.compilejava.net/


import java.util.regex.*;

public class HelloWorld
{
  public static void main(String[] args)
  {
    Pattern p = Pattern.compile("ab(?:a(c))*ac");
     Matcher m = p.matcher("abacacac");
     m.find();
     System.out.println(m.start(1));
  }
}


REPRODUCIBILITY :
This bug can be reproduced always.

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

public class HelloWorld
{
  public static void main(String[] args)
  {
    Pattern p = Pattern.compile("ab(?:a(c))*ac");
     Matcher m = p.matcher("abacacac");
     m.find();
     System.out.println(m.start(1));
  }
}
---------- END SOURCE ----------