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 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 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
