Java - Regular Expressions Regular Expressions

What Is a Regular Expression?

A regular expression describes a pattern in a sequence of characters.

String API

String class has two methods to do the match replacement with Regular Expressions

String replaceAll(String regex, String replacementString):
String replaceFirst(String regex, String replacementString):

Some examples of using the replaceAll() method are as follows:

Demo

public class Main {
  public static void main(String[] args) {
    String regex = ".@.";

    String s = "asdf@book2s.com".replaceAll(regex, "***");
    System.out.println(s);/*from  w w  w .ja  va 2 s. c  om*/
    s = "A@B".replaceAll(regex, "***");
    System.out.println(s);
    s = "A@abc@G".replaceAll(regex, "***");
    System.out.println(s);
    s = "abc%def".replaceAll(regex, "***");
    System.out.println(s);

  }
}

Result

replaceFirst() method replaces the first occurrence of the match.

Demo

public class Main {
  public static void main(String[] args) {
    String regex = ".@.";

    String s = "asdf@book2s.com".replaceFirst(regex, "***");
    System.out.println(s);/*from   w ww .java2 s .co  m*/
    s = "A@B".replaceFirst(regex, "***");
    System.out.println(s);
    s = "A@abc@G".replaceFirst(regex, "***");
    System.out.println(s);
    s = "abc%def".replaceFirst(regex, "***");
    System.out.println(s);

  }
}

Result

Related Topics

• Test Regex method and pattern
• Match line from text file by Scanner and Regex

Exercise

• Write code to check if a string is Integer using regex
• Write code to Convert the string into a regex to allow easy matching.
• Write code to check if a string is Chinese by regex
• Write code to add underscore before any non letter in a string using regex
• Write code to check if a string is Number via regex
• Write code to remove Html Tag using regex
• Write code to get First Positive Number using regex
• Write code to check if a string is Numeric by regex
• Write code to get Host name from a string using regex
• Write code to check if a string is Number using regex
• Write code to Remove all whitespaces and non visible characters such as tab, \n. using regex
• Write code to filter Break Line with regex
• Write code to check if a string is Double using regex
• Write code to get Http File Extension via regex
• truncate After Words using regex
• Write code to remove Blank from a string by regex
• Write code to check if a string is Positive Integer using regular expression
• Write code to give the word count in a given input string.
• Write code to seperate a string On Comma

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