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The start(int group) method of Matcher Class is used to get the start index of the match result already done, from the specified group. Syntax:

public int start(int group)

Parameters: This method takes a parameter group which is the group from which the start index of the matched pattern is required. Return Value: This method returns the index of the first character matched from the specified group. Exception: This method throws:

IllegalStateException if no match has yet been attempted, or if the previous match operation failed.
IndexOutOfBoundsException if there is no capturing group in the pattern with the given index.

Below examples illustrate the Matcher.start() method: Example 1:

Java

// Java code to illustrate start() method

import java.util.regex.*;

public class GFG {
    public static void main(String[] args)
    {

        // Get the regex to be checked
        String regex = "(G*s)";

        // Create a pattern from regex
        Pattern pattern
            = Pattern.compile(regex);

        // Get the String to be matched
        String stringToBeMatched
            = "GeeksForGeeks";

        // Create a matcher for the input String
        Matcher matcher
            = pattern
                  .matcher(stringToBeMatched);

        // Get the current matcher state
        MatchResult result
            = matcher.toMatchResult();
        System.out.println("Current Matcher: "
                           + result);

        while (matcher.find()) {
            // Get the first index of match result
            System.out.println(matcher.start(1));
        }
    }
}

Output:

Current Matcher: java.util.regex.Matcher[pattern=(G*s) region=0,13 lastmatch=] 4 12

Example 2:

Java

// Java code to illustrate start() method

import java.util.regex.*;

public class GFG {
    public static void main(String[] args)
    {

        // Get the regex to be checked
        String regex = "(G*G)";

        // Create a pattern from regex
        Pattern pattern
            = Pattern.compile(regex);

        // Get the String to be matched
        String stringToBeMatched
            = "GFGFGFGFGFGFGFGFGFG";

        // Create a matcher for the input String
        Matcher matcher
            = pattern
                  .matcher(stringToBeMatched);

        // Get the current matcher state
        MatchResult result
            = matcher.toMatchResult();
        System.out.println("Current Matcher: "
                           + result);

        while (matcher.find()) {
            // Get the first index of match result
            System.out.println(matcher.start(0));
        }
    }
}

Output:

Current Matcher: java.util.regex.Matcher[pattern=(G*G) region=0,19 lastmatch=] 0 2 4 6 8 10 12 14 16 18

Reference: Oracle Doc

Matcher start(int) method in Java with Examples

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