Matcher regionStart() method in Java with Examples

The regionStart() method of Matcher Class is used to get the startIndex of the region to be matched by the pattern in the current matcher. This method returns an integer value which is the startIndex of the region of this matcher.

Syntax:

public int regionStart()

Parameters: This method takes no parameters.

Return Value: This method returns a integer value which is the startIndex of the region of this matcher.

Below examples illustrate the Matcher.regionStart() method:

Example 1:

// Java code to illustrate regionStart() method

import java.util.regex.*;

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

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

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

        // Get the String to be matched
        String stringToBeMatched
            = "GeeksForGeeks Geeks for For Geeks Geek";

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

        // Get previous startIndex of region
        // using regionStart() method
        System.out.println("Before changing region, "
                           + " Region starts from: "
                           + matcher.regionStart());

        // Restrict the region to 2, 10
        matcher = matcher.region(2, 10);

        // Get previous startIndex of region
        // using regionStart() method
        System.out.println("After changing region, "
                           + " Region starts from: "
                           + matcher.regionStart());
    }
}

Output:

Before changing region,  Region starts from: 0
After changing region,  Region starts from: 2

Example 2:

// Java code to illustrate regionStart() method

import java.util.regex.*;

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

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

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

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

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

        // Get previous startIndex of region
        // using regionStart() method
        System.out.println("Before changing region, "
                           + " Region starts from: "
                           + matcher.regionStart());

        // Restrict the region to 5, 10
        matcher = matcher.region(5, 10);

        // Get previous startIndex of region
        // using regionStart() method
        System.out.println("After changing region, "
                           + " Region starts from: "
                           + matcher.regionStart());
    }
}

Output:

Before changing region,  Region starts from: 0
After changing region,  Region starts from: 5

Reference: https://docs.oracle.com/javase/9/docs/api/java/util/regex/Matcher.html#regionStart–