Matcher replaceFirst(Function) method in Java with Examples

The replaceFirst(Function) method of Matcher Class behaves as a append-and-replace method. This method replaces first instance of the pattern matched in the matcher with the function passed in the parameter.

Syntax:

public String replaceFirst(
        Function replacerFunction)

Parameters: This method takes a parameter replacerFunction which is the function that defines the replacement of the matcher.

Return Value: This method returns a String with the target String constructed by replacing the String.

Exceptions: This method throws following exceptions:

• NullPointerException: if the replacer function is null
• ConcurrentModificationException: if it is detected that the replacer function modified this matcher’s state

Below examples illustrate the Matcher.replaceFirst() method:

Example 1:

// Java code to illustrate replaceFirst() 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);

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

        // Get the String to be replaced
        String stringToBeReplaced = "Geeks";
        StringBuilder builder
            = new StringBuilder();

        // Replace every matched pattern
        // with the target String
        // using replaceFirst() method
        System.out.println("After Replacement: "
                           + matcher
                                 .replaceFirst(
                                     x -> x.group().toUpperCase()));
    }
}

Output:

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

Example 2:

// Java code to illustrate replaceFirst() method

import java.util.regex.*;

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

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

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

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

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

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

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

        // Replace every matched pattern
        // with the target String
        // using replaceFirst() method
        System.out.println("After Replacement: "
                           + matcher
                                 .replaceFirst(
                                     x -> x.group().toLowerCase()));
    }
}

Output:

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

Reference: https://docs.oracle.com/javase/9/docs/api/java/util/regex/Matcher.html#replaceFirst-java.util.function.Function-