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 contructed 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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