java - Regex searches beyond string boundary -

the code 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 " + 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 6, string's highest index 5.
why there match @ index 6? thank in advance!

you have 6 indices returned because there 6 matches here since \d* can match empty string. there empty string before each character in input string, because regex engine processing text @ each position looking boundaries or specific characters.

here visualization:

here, engine examines beginning of string, , says: "i see no digit, can return match, since number of digits can 0". returns empty string match, , goes on b. , on until end of string.

if need find numbers, use + quantifier \d shorthand class.

see ideone demo