Explain the Java regular expression construct "re?".

The subexpression/metacharacter “re?” matches 0 or 1 occurrence of the preceding expression.

Example 1

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegexExample {
   public static void main( String args[] ) {
      String regex = "Wel?";
      String input = "Welcome to Tutorialspoint";
      Pattern p = Pattern.compile(regex);
      Matcher m = p.matcher(input);
      int count = 0;
      while(m.find()) {
         count++;
      }
      System.out.println("Number of matches: "+count);
   }
}

Output

Number of matches: 1

Example 2

Following Java program accepts a string from the user, verifies whether it contains alphabet (both cases), It also accepts digits.

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Test {
   public static void main( String args[] ) {
      String regex = "[a-zA-Z][0-9]?";
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter an input string: ");
      String input = sc.nextLine();
      //Creating a Pattern object
      Pattern p = Pattern.compile(regex);
      //Creating a Matcher object
      Matcher m = p.matcher(input);
      if(m.find()) {
         System.out.println("Match occurred");
      } else {
         System.out.println("Match not occurred");
      }
   }
}

Output 1

Enter an input string:
sample text
Match occurred

Output 2

Enter an input string:
sample text 34 56
Match occurred

Output 3

Enter an input string:
32 89 45 63
Match not occurred