Explain Regular Expression "A" Metacharacter in Java

The subexpression/metacharacter “\A” matches the beginning of the entire string.

Example 1

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegexExample {
   public static void main( String args[] ) {
      String regex = "\AHi";
      String input = "Hi how are you 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

Example2

Following Java program accepts a string from the user verifies whether it contains non-ASCII characters.

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class StartingOfInput {
   public static void main( String args[] ) {
      String regex = "\A\p{ASCII}*\z";
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter the 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("Given input contains only ASCII characters ");
      } else {
         System.out.println("Given input contains non-ASCII characters ");
      }
   }
}

Output1

Enter the input string:
What is your name
Given input contains only ASCII characters

Output2

Enter the input string:
whÿ do we fall
Given input contains non-ASCII characters