Java String replaceAll() replaces all the occurrences of a particular character, string or a regular expression in the string. The method returns a new string as its output. The method requires two parameters.
String replaceAll() Syntax
public String replaceAll(String regex, String replacement)
A regex that represents the pattern to look for while replacing and a replacement string to replace it with.
There are multiple types of substitutions that are possible with String replaceAll() method.
Replacing a single character
To replace a single character, specify the character to be replaced as the regex.
public class Main {
public static void main(String[] args) {
String s = "Welcome to JournalDev";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceAll("o","@");
System.out.println("New String: "+rep);
}
}

Original String: Welcome to JournalDev
New String: Welc@me t@ J@urnalDev
Replace sequence of characters
To replace a sequence of characters, mention the sequence as the regex in the replaceAll() function.
public class Main {
public static void main(String[] args) {
String s = "Welcome to JournalDev";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceAll("to","this is");
System.out.println("New String: "+rep);
}
}

Original String : Welcome to JournalDev
New String: Welcome this is JournalDev
Remove/Replace Spaces
The replaceAll() method can remove or replace the whitespaces in your text string.
public class Main {
public static void main(String[] args) {
String s = "Welcome to JournalDev";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceAll(" ","");
System.out.println("New String: "+rep);
}
}

Original String : Welcome to JournalDev
New String: WelcometoJournalDev
Similarly, to replace the whitespaces:
public class Main {
public static void main(String[] args) {
String s = "Welcome to JournalDev";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceAll(" ","_");
System.out.println("New String: "+rep);
}
}

Original String : Welcome to JournalDev
New String: Welcome_to_JournalDev
Hiding numeric digits in a text
The replaceAll() can hide numeric digits that appear in a string. The digits can be replaced by ‘*’.
public class Main {
public static void main(String[] args) {
String s = "Hello! this is the number : 1234 ";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceAll("[0-9]","*");
System.out.println("New String: "+rep);
}
}

Original String : Hello! this is the number : 1234
New String: Hello! this is the number : ****
Creating custom messages
replaceAll() can replace the strings in a generic string to generate a custom message. Let’s take values from a map and generate a custom message.
import java.util.HashMap;
import java.util.Map;
public class Main {
public static void main(String[] args) {
String s = "dear #Employee#, your Current Salary is #Salary#.";
Map<String, String> vals = new HashMap<String, String>();
vals.put("#Employee#", "John");
vals.put("#Salary#", "12000");
for (String key : vals.keySet()) {
s = s.replaceAll(key, vals.get(key));
}
System.out.println(s);
}
}

dear John, your Current Salary is 12000.
Replace First
Unlike replceAll(), replaceFirst() only replaces the first instance of the pattern.
public class Main {
public static void main(String[] args) {
String s = "Welcome to JournalDev";
System.out.println("Original String : "+s);
System.out.println();
String rep = s.replaceFirst("o","@");
System.out.println("New String: "+rep);
}
}

Original String: Welcome to JournalDev
New String: Welc@me to JournalDev
Conclusion
The replaceAll() method replaces a regex with a string. This is different from the simple replace() method that only performs character substitutions. You can learn more about the repalceAll() function from its official documentation.
Thank you 馃檪
Very nice example! Thanks.