Question

Can anyone explain:

1. Why the two patterns used below give different results? (answered below)
2. Why the 2nd example gives a group count of 1 but says the start
   and end of group 1 is -1?

 public void testGroups() throws Exception
 {
  String TEST_STRING = "After Yes is group 1 End";
  {
   Pattern p;
   Matcher m;
   String pattern="(?:Yes|No)(.*)End";
   p=Pattern.compile(pattern);
   m=p.matcher(TEST_STRING);
   boolean f=m.find();
   int count=m.groupCount();
   int start=m.start(1);
   int end=m.end(1);

   System.out.println("Pattern=" + pattern + "\t Found=" + f + " Group count=" + count + 
     " Start of group 1=" + start + " End of group 1=" + end );
  }

  {
   Pattern p;
   Matcher m;

   String pattern="(?:Yes)|(?:No)(.*)End";
   p=Pattern.compile(pattern);
   m=p.matcher(TEST_STRING);
   boolean f=m.find();
   int count=m.groupCount();
   int start=m.start(1);
   int end=m.end(1);

   System.out.println("Pattern=" + pattern + "\t Found=" + f + " Group count=" + count + 
     " Start of group 1=" + start + " End of group 1=" + end );
  }

 }

Which gives the following output:

Pattern=(?:Yes|No)(.*)End  Found=true Group count=1 Start of group 1=9 End of group 1=21
Pattern=(?:Yes)|(?:No)(.*)End  Found=true Group count=1 Start of group 1=-1 End of group 1=-1

Answer 1

To summarise,

1) The two patterns give different results because of the precedence rules of the operators.

• (?:Yes|No)(.*)End matches (Yes or
  No) followed by .*End
• (?:Yes)|(?:No)(.*)End matches (Yes)
  or (No followed by .*End)

2) The second pattern gives a group count of 1 but a start and end of -1 because of the (not necessarily intuitive) meanings of the results returned by the Matcher method calls.

• Matcher.find() returns true if a match was found. In your case the match was on the (?:Yes) part of the pattern.
• Matcher.groupCount() returns the number of capturing groups in the pattern regardless of whether the capturing groups actually participated in the match. In your case only the non capturing (?:Yes) part of the pattern participated in the match, but the capturing (.*) group was still part of the pattern so the group count is 1.
• Matcher.start(n) and Matcher.end(n) return the start and end index of the subsequence matched by the n th capturing group. In your case, although an overall match was found, the (.*) capturing group did not participate in the match and so did not capture a subsequence, hence the -1 results.

3) (Question asked in comment.) In order to determine how many capturing groups actually captured a subsequence, iterate Matcher.start(n) from 0 to Matcher.groupCount() counting the number of non -1 results. (Note that Matcher.start(0) is the capturing group representing the whole pattern, which you may want to exclude for your purposes.)