Matcher replaceFirst(String) Method in Java with Examples

The replaceFirst(String) method of the Matcher class is used to replace only the first substring that matches a regular expression with a specified replacement string. Unlike replaceAll(), it stops after replacing the first match and returns a new string.

• Replaces only the first occurrence of the matched pattern.
• Returns a new string while leaving the original string unchanged.
• Useful when only the first matching substring needs to be updated.

Example: Replacing the First Occurrence of a Word

import java.util.regex.*;

public class Geeks {

    public static void main(String[] args) {

        // Input string
        String text = "Java is easy. Java is powerful.";

        // Create pattern and matcher
        Pattern pattern = Pattern.compile("Java");
        Matcher matcher = pattern.matcher(text);

        System.out.println("Before Replacement: " + text);

        // Replace the first occurrence
        String result = matcher.replaceFirst("Python");

        System.out.println("After Replacement: " + result);
    }
}

Before Replacement: Java is easy. Java is powerful.
After Replacement: Python is easy. Java is powerful.

Explanation: The regular expression matches the word "Java". The replaceFirst() method replaces only the first occurrence with "Python", while the second occurrence remains unchanged.

Syntax

public String replaceFirst(String replacement)

• 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: Replacing the First Number

import java.util.regex.*;

public class Geeks {

    public static void main(String[] args) {

        // Input string
        String text = "Order123 and Order456 are ready.";

        // Create pattern and matcher
        Pattern pattern = Pattern.compile("\\d+");
        Matcher matcher = pattern.matcher(text);

        System.out.println("Before Replacement: " + text);

        // Replace the first sequence of digits
        String result = matcher.replaceFirst("###");

        System.out.println("After Replacement: " + result);
    }
}

Before Replacement: Order123 and Order456 are ready.
After Replacement: Order### and Order456 are ready.

Explanation: The regular expression \\d+ matches one or more digits. The replaceFirst() method replaces only the first matched number (123) with ###, leaving the second number (456) unchanged.

Difference Between replaceFirst() and replaceAll()

Advantages of replaceFirst(String) Method

• Replaces only the first matching substring.
• Returns a new string without modifying the original string.
• Supports regular expressions for flexible pattern matching.
• Useful for updating only the first occurrence in a string.
• Simple and efficient for one-time replacement operations.