String's replace operation

Java's String class provides four replace methods: replace(char, char), replace(CharSequence, CharSequence), replaceAll(regex, replacement), and replaceFirst(regex, replacement). This article explains the difference between literal and regex-based replacement, how to use backreferences in replaceAll(), and why replaceAll() treats its first argument as a regex while replace() does not.

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In this article, we are going to look at how to replace a character or a substring within a String.

Contents

Replace all occurrences of a specific character from a string

We can use String.replace(char oldChar, char newChar) method to replace a specific character passed in 1st argument oldChar with a replacement character newChar passed as 2nd argument.

For example, in the below example, we will replace 'W' letter with lowercase 'w' in "Hello World" string.

Java
public class ReplaceString {

  public static void main(String[] args) {
      String s1 = "Hello World";
      String afterReplacingW = s1.replace('W', 'w');
      System.out.println("Before replacing W : "+s1);
      System.out.println("After replacing W with lowercase w : "+ afterReplacingW);
  }
}

When you run the above program, you will get the below output.

Output
Before replacing W : Hello World 
      After replacing W with lowercase w : Hello world

Note: Note that, String.replace(char oldChar, char newChar) will replace all occurrences of oldChar with newChar.

Replace all occurrences of a substring from a string

We can use String.replace(CharSequence target, CharSequence replacement) method to replace the target substring with the replacement string.

For example, in the below example, we will replace the substring "World" with "CsCode" in "Hello World" string.

Java
public class ReplaceString {

  public static void main(String[] args) {
      String s2 = "Hello World";
      String afterReplacingWorld = s2.replace("World", "CsCode");
      System.out.println("Before replacing World : "+s2);
      System.out.println("After replacing World with CsCode word : "+ afterReplacingWorld);
  }
}

When you run the above program, you will get the below output.

Output
Before replacing World : Hello World 
    After replacing World with CsCode word : Hello CsCode

Note: Note that, String.replace(CharSequence target, CharSequence replacement) will replace all occurrences of target with replacement.

Replace matching pattern substrings from a string

We can use String.replaceAll(String regex, String replacement) method to replace the matching patterns using regex and replace them with replacement string.

For example, in the below example, we will replace space characters " " with "%20" in the string "url with spaces".

Java
public class ReplaceString {

  public static void main(String[] args) {
      String s3 = "url with spaces";
      String afterApplyingRegEx = s3.replaceAll(" ", "%20");
      System.out.println("Before formatting url : "+s3);
      System.out.println("After formatting url : "+ afterApplyingRegEx);

  }
}

When you run the above program, you will get the below output.

Output
Before formatting url : url with spaces 
    After formatting url : url%20with%20spaces
 Note that String.replaceAll method will replaces all occurrences of matching pattern using the regular expression, but if you want to replace only first matching occurrance, then you can use String.replaceFirst(String regex, String replacement) method.

Above example complete source code can be found at Github