您可以使用元字符“ \\ d ”或使用以下表達式來匹配給定字符串中的數字：

[0-9]

例子1

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Example {
   public static void main(String args[]) {
      //從用戶讀取字符串
      System.out.println("Enter a String");
      Scanner sc = new Scanner(System.in);
      String input = sc.nextLine();
      String regex = "\\d";
      //編譯正則表達式
      Pattern pattern = Pattern.compile(regex);
      //檢索匹配器對象
      Matcher matcher = pattern.matcher(input);
      int count = 0;
      while(matcher.find()) {
         count++;
      }
      System.out.println("Number of digits: "+count);
   }
}

輸出結果

Enter a String
sample text 1234 6657
Number of digits: 8

例子2

import java.util.Scanner;
public class RegexExample {
   public static void main( String args[] ) {
      //接受10位數字的正則表達式
      String regex = "\\d{10}";
      System.out.println("Enter input value: ");
      Scanner sc = new Scanner(System.in);
      String input = sc.nextLine();
      boolean result = input.matches(regex);
      if(result) {
         System.out.println("10 digit number");
      } else {
         System.out.println("wrong input");
      }
   }
}

輸出1

Enter input value:
9848022558
10 digit number

輸出2

Enter input value:
5337
wrong input