Pattern compile() method in Java with Examples

The pattern class of the java.regex package is a compiled representation of a regular expression.

The compile() method of this class accepts a string value representing a regular expression and returns a Pattern object.

Example

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class CompileExample {
   public static void main( String args[] ) {
      //Reading string value
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter input string");
      String input = sc.nextLine();
      //Regular expression to find digits
      String regex = "(\\d)";
      //Compiling the regular expression
      Pattern pattern = Pattern.compile(regex);
      //Printing the regular expression
      System.out.println("Compiled regular expression: "+pattern.toString());
      //Retrieving the matcher object
      Matcher matcher = pattern.matcher(input);
      //verifying whether match occurred
      if(matcher.find()) {
         System.out.println("Given String contains digits");
      } else {
         System.out.println("Given String does not contain digits");
      }
   }
}

Output

Enter input string
hello my id is 1120KKA
Compiled regular expression: (\d)
Given String contains digits

Another variant of this method accepts an integer value representing flags, where each flag specifies an optional condition, for example, CASE_INSENSITIVE ignores the case while compiling the regular expression.

Example

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class CompileExample {
   public static void main( String args[] ) {
      //Compiling the regular expression
      Pattern pattern = Pattern.compile("[t]", Pattern.CASE_INSENSITIVE);
      //Retrieving the matcher object
      Matcher matcher = pattern.matcher("Tutorialspoint");
      int count = 0;
      while(matcher.find()) {
         count++;
      }
      System.out.println("Number of matches: "+count);
   }
}

Output

Enter input string
Tutorialspoint
Number of matches: 3