Regular expression["0-9"] issue in java

Question

I have a text file as follows:

---

Title
XYZ
Id name
1  abc
2  pqr
3  xyz

---

I need to read the content starting with the integer value and I used the regular expression as in the following code.

public static void main(String[] args) throws FileNotFoundException {
   FileInputStream file= new FileInputStream("C:\\Users\\ap\\Downloads\\sample1.txt"); 
   Scanner scanner = new Scanner(file);
          while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            if (line.startsWith("[0-9]")) {
                System.out.println("Line: "+line);
            }
           }
}

The above code can't detect the lines starting with integers. However, it works fine if single integer values are passed to startsWith() function. Please suggest, where I went wrong.

Answer 1

String#startsWith(String) method doesn't take regex. It takes a string literal.

To check the first character is digit or not, you can get the character at index 0 using String#charAt(int index) method. And then test that character is digit or not using Character#isDigit(char) method:

if (Character.isDigit(line.charAt(0)) {
    System.out.println(line);
}

Answer 2

For regex you can use the "matches" method, like this:

line.matches("^[0-9].*")