Regex

[Varnam Aayiram]

Hi Folks,

I have a regex related question:

Following is the code:

import java.util.regex.*;

class Regex 
{
	public static void main(String [] args) 
	{
		Pattern p = Pattern.compile(args[0]);
		Matcher m = p.matcher(args[1]);
		System.out.println("Pattern is " + m.pattern());

		while(m.find()) 
		{
			System.out.println(m.start() + " " + m.group());
		}
	}

}

following is the invocation:

% java Regex "a?" "aba"




and the following is the output:

Pattern is a?
0 a
1
2 a
3

How is 3 even possible? I thought the index should read as ("0,1,2"). Hope someone can advise. Thanks.

[Jelle Klap]

Have a look at this SCJP FAQ entry, which explains this quite nicely.
The cliff notes version: the ? in the regex pattern is a greedy quantifier, which matches zero-length occurances of the preceeding pattern segment - in this case a zero-length occurance of "a" at index 3.
The fact that the index returned is beyond what you'd expect possible give the length of te String adds to the confusion a bit, but that's related to how the Matcher processes the input.
More info on zero-length matches here.

[Varnam Aayiram]

Thanks Jelle Klap