String replaceAll() method - Syntax and Examples

Learn how to use the Java String replaceAll() method. This tutorial covers syntax, parameters, return value, and practical examples.

This reference explains String replaceAll() method - Syntax and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

The replaceAll() method in Java is a powerful tool for replacing all occurrences of a pattern within a string. Unlike the replace() method, which only replaces literal characters, replaceAll() uses regular expressions to identify and replace patterns.

    public String replaceAll(String regex, String replacement)
  

This method returns a new string with all occurrences of the specified pattern replaced by the given replacement string.

Let's start with a simple example. This demonstrates how to replace all spaces in a string with underscores.

    String str = "This is a sample string";
String replacedStr = str.replaceAll("\s", "_");
System.out.println(replacedStr);
    
  

This_is_a_sample_string

Explanation: The regex "\s" matches any whitespace character (space, tab, newline, etc.). We replace each occurrence with an underscore.

Here's how to remove all digits from a string using the replaceAll() method. Imagine you have a product code and want to clean it up by removing numeric characters.

    String str = "ProductCode123";
String cleanedStr = str.replaceAll("[0-9]", "");
System.out.println(cleanedStr);
    
  

ProductCode

Explanation: The regex "[0-9]" matches any digit (0 through 9). We replace each digit with an empty string, effectively removing them.

This example shows how to remove HTML tags from a string. This is useful for sanitizing user-generated content or extracting text from web pages.

    String html = "<p>This is some HTML.</p>";
String plainText = html.replaceAll("<[^>]*>", "");
System.out.println(plainText);
    
  

This is some HTML.

Explanation: The regex "<[^>]*>" matches any HTML tag (e.g., <p>, <b>). It works by matching the opening bracket (<), then any character that isn't a closing bracket (>) zero or more times ([^>]*), and finally the closing bracket (>). We replace each tag with an empty string.

You can use the pipe symbol (|) in your regular expression to match multiple patterns. This example replaces both 'apple' and 'banana' with 'fruit'.

    String str = "I like apple and banana";
String replacedStr = str.replaceAll("apple|banana", "fruit");
System.out.println(replacedStr);
    
  

I like fruit and fruit

Explanation: The regex "apple|banana" matches either 'apple' or 'banana'. The pipe symbol (|) acts as an "or" operator in regular expressions.

Capturing groups allow you to refer back to the matched text within your replacement string. This example replaces dates formatted as MM/DD/YYYY with a standardized format of YYYY-MM-DD.

        String dateString = "Date: 12/25/2023";
String replacedDateString = dateString.replaceAll("(\d{2})/(\d{2})/(\d{4})", "$3-$1-$2");
System.out.println(replacedDateString);
        
    

Date: 2023-12-25

Explanation: The regex "(\d{2})/(\d{2})/(\d{4})" captures three groups of digits. The first group ((\d{2})) matches the month (two digits), the second group ((\d{2})) matches the day, and the third group ((\d{4})) matches the year. The replacement string uses $3-$1-$2 to rearrange these captured groups into a new format.

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Related explanations: Java Data Types - Primitive and Non-Primitive, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide.