Pattern pattern() method in Java with examples

The java.util.regex package of java provides various classes to find particular patterns in character sequences. The pattern class of this package is a compiled representation of a regular expression.

The pattern() method of the Pattern class fetches and returns the regular expression in the string format, using which the current pattern was compiled.

Example 1

import java.util.regex.Pattern;
public class PatternExample {
   public static void main(String[] args) {
      String date = "12/09/2019";
      String regex = "^(1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])/[0-9]{4}$";
      //Creating a pattern object
      Pattern pattern = Pattern.compile(regex);
      if(pattern.matcher(date).matches()) {
         System.out.println("Date is valid");
      } else {
         System.out.println("Date is not valid");
      }
      //Retrieving the regular expression of the current pattern
      String regularExpression = pattern.pattern();
      System.out.println("Regular expression: "+regularExpression);
   }
}

Output

Date is valid
Regular expression: ^(1[0-2]|0[1-9])/(3[01]|[12][0-9]|0[1-9])/[0-9]{4}$

Example 2

public class PatternExample {
   public static void main(String[] args) {
      String input = "Hi my id is 056E1563";
      //Regular expression using groups
      String regex = "(.*)?(\d+)";
      //Creating a pattern object
      Pattern pattern = Pattern.compile(regex);
      if(pattern.matcher(input).matches()) {
         System.out.println("Match found");
      } else {
         System.out.println("Match not found");
      }
      //Retrieving the regular expression of the current pattern
      String regularExpression = pattern.pattern();
      System.out.println("Regular expression: "+regularExpression);
   }
}

Output

Match found
Regular expression: (.*)?(\d+)