Question

My code :

Pattern pattern = Pattern.compile("a?");
Matcher matcher = pattern.matcher("ababa");
while(matcher.find()){
   System.out.println(matcher.start()+"["+matcher.group()+"]"+matcher.end());
}

Output :

0[a]1
1[]1
2[a]3
3[]3
4[a]5
5[]5

What I know :

• “a?” stands for zero or one occurrence of the character ‘a’.

Java API says :

• matcher.start() returns the start index of the previous match.
• matcher.end() returns the offset after the last character matched.
• matcher.group() returns the input subsequence matched by the previous
  match. For a matcher m with input sequence s, the expressions
  m.group() and s.substring(m.start(), m.end()) are equivalent. And for
  some patterns, for example a*, match the empty string. This method
  will return the empty string when the pattern successfully matches
  the empty string in the input.

What I want to know:

1. In which situations does the regex engine encounters a zero
   occurrence of a given character(s) – Here for character ‘a’.
2. In those situation what are values actually returns by the start(),
   end() and group() methods in the matcher. I have mentioned what the
   java API said. But I’m little unclear when it comes to the practical
   situation as above.

Answer 1

The ? is a greedy quantifier, therefore it will first try to match the 1-occurence before trying the 0-occurence. In you string,

1. it starts with the first char ‘a’ and tries to match agains the 1-occurence. The ‘a’ char matches and so it returns the first result you see
2. then it moves forward and find a ‘b’. The ‘b’ char does not match your regexp 1-occurence, so the engine backtracks and attempt to match a 0-occurence. Result is that the empty string is matched–> you get your second result.
3. then it moves ahead of b since no more matches are possible there and it starts again with your second ‘a’ char.
4. etc… you get the point…

It is a bit more complicated than that but that is the main idea. When the 1-occurence cannot match, it will then try with the 0-occurence.

As for the values of start, end and group, they will be where the match starts, ends and the group is what has been matched, so in the first 0-occurence match of your string, you get 1, 1 and the emtpy string. I am not sure this really answers your question.