Java - replace first 3 characters in string

In this article, we would like to show you how to replace the first 3 characters in string in Java.

Quick solution:

String text = "ABCD";
String replacement = "xyz";
String result = replacement + text.substring(3);

System.out.println(result);  // xyzD

or:

String text = "ABCD";
String replacement = "xyz";
String result = replacement.concat(text.substring(3));

System.out.println(result);  // xyzD

or:

String text = "ABCD";
String replacement = "xyz";
String result = text.replaceAll("^.{3}", replacement);

System.out.println(result);  // xyzD

1. Practical example using String substring()

1.1 With + operator

In this example, we remove last 3 characters from the original string and add the replacement at the beginning.

public class Example {

    public static void main(String[] args) {
        String text = "ABCD";
        String replacement = "xyz";
        String result = replacement + text.substring(3);

        System.out.println(result);  // xyzD
    }
}

1.2 With concat() method

This approach is equivalent to the above one. Instead of + operator we concatenate the strings using concat() method.

public class Example {

    public static void main(String[] args) {
        String text = "ABCD";
        String replacement = "xyz";
        String result = replacement.concat(text.substring(3));

        System.out.println(result);  // xyzD
    }
}

2. Practical example using String replaceAll() with regex pattern

In this example, we use string replaceAll() with "^.{3}" regex to replace the first 3 characters in the text string.

Regex explanation:

• ^ - matches the beginning of the string,
• . - matches any character except linebreaks,
• {3} - matches the specified quantity of the previous token (in our case the .).

public class Example {

    public static void main(String[] args) {
        String text = "ABCD";
        String replacement = "xyz";
        String result = text.replaceAll("^.{3}", replacement);

        System.out.println(result);  // xyzD
    }
}

Note:

Regular expressions are slower than substring() method.