Regex searches beyond string boundary

Question

The code is given below:

import java.util.regex.*;

public class RegEx {

    public static void main(String[] args) {

        Pattern p = Pattern.compile("\\d*");
        Matcher m = p.matcher("ab56ef");
        System.out.println("Pattern is " + m.pattern());
        while (m.find()) {
            System.out.print("index: " + m.start() + " " + m.group());
        }
    }
}

The result is:

index: 0 index: 1 index: 2 56 index: 4 index: 5 index: 6

Since "ab34ef" length is 6, the string's highest index is 5.
Why is there a match at index 6? Thank you in advance!

`\d*` matches zero or more digits, so this would match also the empty string which exists before each non-matching character. — Avinash Raj, Jun 03 '15 at 07:21
@Kasra: Without a visualization, that would not be clear to newbies. — Wiktor Stribiżew, Jun 03 '15 at 07:27
@stribizhev Don't worry,but you could attached the link to Avinash's comment as another comment ;) any way good answer and you have got my up vote already ;)! — Mazdak, Jun 03 '15 at 07:30
@Kasra, no, that is not the one, that post is about how capturing groups work. This question is a kind of a subset of the question you found. — Wiktor Stribiżew, Jun 03 '15 at 08:04

Wiktor Stribiżew · Answer 1 · 2015-06-03T07:30:19.873

You have 6 indices returned because there are 6 matches here since \d* can match an empty string. There is always an empty string before each character in an input string, because the regex engine is processing text at each position looking for boundaries or specific characters.

Here is the visualization:

enter image description here

Here, the engine examines the beginning of a string, and says: "I see no digit, but I can return a match, since the number of digits can be 0". It returns the empty string as a match, and goes on to b. And so on until the end of string.

If you need to find all numbers, just use a + quantifier with \d shorthand class.

See IDEONE demo

Please consider accepting/upvoting if it proved working/helpful to you. — Wiktor Stribiżew, Jul 11 '15 at 07:09

Regex searches beyond string boundary

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