Java String replaceAll()

class Main {
  public static void main(String[] args) {
    String str1 = "Java123is456fun";

    // regex for sequence of digits
    String regex = "\\d+";

    // replace all occurrences of numeric
    // digits by a space
    System.out.println(str1.replaceAll(regex, " "));


  }
}

// Output: Java is fun

Syntax of replaceAll()

The syntax of the replaceAll() method is:

string.replaceAll(String regex, String replacement)

Here, string is an object of the String class.

replaceAll() Parameters

The replaceAll() method takes two parameters.

• regex - a regex (can be a typical string) that is to be replaced
• replacement - matching substrings are replaced with this string

replaceAll() Return Value

The replaceAll() method

• returns a new string where each occurrence of the matching substring is replaced with the replacement string.

Example 1: Java String replaceAll()

class Main {
  public static void main(String[] args) {
    String str1 = "aabbaaac";
    String str2 = "Learn223Java55@";

    // regex for sequence of digits
    String regex = "\\d+";

    // all occurrences of "aa" is replaceAll with "zz"
    System.out.println(str1.replaceAll("aa", "zz"));  // zzbbzzac


    // replace a digit or sequence of digits with a whitespace
    System.out.println(str2.replaceAll(regex, " "));  // Learn Java @


  }
}

In the above example, "\\d+" is a regular expression that matches one or more digits.

Escaping Characters in replaceAll()

The replaceAll() method can take a regex or a typical string as the first argument. It is because a typical string in itself is a regex.

In regex, there are characters that have special meaning. These metacharacters are:

\ ^ $ . | ? * + {} [] ()

If you need to match substring containing these metacharacters, you can either escape these characters using \ or use the replace() method.

// Program to replace the + character
class Main {
  public static void main(String[] args) {
    String str1 = "+a-+b";

    // replace "+" with "#" using replaceAll()
    // need to escape "+"
    System.out.println(str1.replaceAll("\\+", "#"));  // #a-#b


    // replace "+" with "#" using replace()
    System.out.println(str1.replace("+", "#"));  // #a-#b

  }
}

As you can see, when we use the replace() method, we do not need to escape metacharacters. To learn more, visit: Java String replace().

If you need to replace only the first occurrence of the matching substring, use the Java String replaceFirst() method.