Matcher replaceFirst(String) Method in Java with Examples

The replaceFirst() method of Matcher Class behaves as an append-and-replace method. This method reads the input string and replaces it with the first matched pattern in the matcher string. 

Syntax:

public String replaceFirst(String stringToBeReplaced)

Parameters: The string to be replaced that is the String to be replaced in the matcher.

Return Type: A string with the target string constructed by replacing the string.

Example 1:

Java

// Java Program to Illustrate replaceFirst() Method
// of Matcher class

// Importing required classes
import java.util.regex.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Getting the regex to be checked
        String regex = "(Geeks)";

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

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

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

        // Displaying the string to be matched
        // before replacing
        System.out.println("Before Replacement: "
                           + stringToBeMatched);

        // Getting the string to be replaced
        String stringToBeReplaced = "GFG";
        StringBuilder builder = new StringBuilder();

        // Replacing every matched pattern with the target
        // string using replaceFirst() method
        System.out.println(
            "After Replacement: "
            + matcher.replaceFirst(stringToBeReplaced));
    }
}

Output:

Before Replacement: GeeksForGeeks Geeks for For Geeks Geek
After Replacement: GFGForGeeks Geeks for For Geeks Geek

Example 2:

Java

// Java Program to Illustrate replaceFirst() Method

// Importing required classes
import java.util.regex.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Getting the regex to be checked
        String regex = "(FGF)";

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

        // Getting the string to be matched
        String stringToBeMatched = "FGF FGF FGF FGF";

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

        // Printing string on console
        // before replacement
        System.out.println("Before Replacement: "
                           + stringToBeMatched);

        // Getting the string to be replaced
        String stringToBeReplaced = "GFG";
        StringBuilder builder = new StringBuilder();

        // Replacing every matched pattern with target
        // string using replaceFirst() method and printing
        // the string to be replaced
        System.out.println(
            "After Replacement: "
            + matcher.replaceFirst(stringToBeReplaced));
    }
}

Output:

Before Replacement: FGF FGF FGF FGF
After Replacement: GFG FGF FGF FGF