Matcher replaceFirst(Function) method in Java with Examples

Last Updated : 01 Jul, 2020

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-

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