CODEWITHSUNDEEP
javaregexstring
sevenpi
sevenpi

Reputation: 23

Why does regex="" (an empty pattern) match at every character position?

I have regex="" and a String str="stackoveflow";

I don't understand why it is matching every character in the string. can you explain to me?

public class test {

    public static void main(String[] args){
        Console console = System.console();
        String str="stackoveflow";          
        Pattern pattern = Pattern.compile("");
        Matcher matcher = pattern.matcher(str);
        while (matcher.find()) {
        console.format("I found the text" +
            " \"%s\" starting at " +
            "index %d and ending at index %d.%n",
            matcher.group(),
            matcher.start(),
            matcher.end());     
        }
    }
}

Output is:

I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.
I found the text "" starting at index 2 and ending at index 2.
I found the text "" starting at index 3 and ending at index 3.
I found the text "" starting at index 4 and ending at index 4.
I found the text "" starting at index 5 and ending at index 5.
I found the text "" starting at index 6 and ending at index 6.
I found the text "" starting at index 7 and ending at index 7.
I found the text "" starting at index 8 and ending at index 8.
I found the text "" starting at index 9 and ending at index 9.
I found the text "" starting at index 10 and ending at index 10.
I found the text "" starting at index 11 and ending at index 11.
I found the text "" starting at index 12 and ending at index 12.

Upvotes: 1

Views: 556

Answers (1)

Stephen C
Stephen C

Reputation: 718788

Pattern("") matches a string consisting of zero characters. You can find one of those at every position in the string.

Note: if you changed find to match, you should find that there are no matches. (With match the pattern needs to match the entire input, and the entire input does not match a sequence of zero characters.)

Before you edited the question, your pattern was Pattern("e*"). That means zero or more repetitions of the character 'e'. By the logic above, you can "find" one of those at every character position in the input.

Upvotes: 2

Related Questions