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:

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// 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()));
    }
}

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Output:


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



Example 2:













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// 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())); 


    } 


} 



















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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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