How do I find out if first character of a string is a number?

In Java is there a way to find out if first character of a string is a number?

One way is

string.startsWith("1")

and do the above all the way till 9, but that seems very inefficient.

Character.isDigit(string.charAt(0))

Note that this will allow any Unicode digit, not just 0-9. You might prefer:

char c = string.charAt(0);
isDigit = (c >= '0' && c <= '9');

Or the slower regex solutions:

s.substring(0, 1).matches("\\d")
// or the equivalent
s.substring(0, 1).matches("[0-9]")

However, with any of these methods, you must first be sure that the string isn't empty. If it is, charAt(0) and substring(0, 1) will throw a StringIndexOutOfBoundsException. startsWith does not have this problem.

To make the entire condition one line and avoid length checks, you can alter the regexes to the following:

s.matches("\\d.*")
// or the equivalent
s.matches("[0-9].*")

If the condition does not appear in a tight loop in your program, the small performance hit for using regular expressions is not likely to be noticeable.

Regular expressions are very strong but expensive tool. It is valid to use them for checking if the first character is a digit but it is not so elegant :) I prefer this way:

public boolean isLeadingDigit(final String value){
    final char c = value.charAt(0);
    return (c >= '0' && c <= '9');
}

IN KOTLIN :

Suppose that you have a String like this :

private val phoneNumber="9121111111"

At first you should get the first one :

val firstChar=phoneNumber.slice(0..0)

At second you can check the first char that return a Boolean :

firstChar.isInt() // or isFloat()

regular expression starts with number->'^[0-9]' 
Pattern pattern = Pattern.compile('^[0-9]');
 Matcher matcher = pattern.matcher(String);

if(matcher.find()){

System.out.println("true");
}

I just came across this question and thought on contributing with a solution that does not use regex.

In my case I use a helper method:

public boolean notNumber(String input){
	boolean notNumber = false;
	try {
		// must not start with a number
		@SuppressWarnings("unused")
		double checker = Double.valueOf(input.substring(0,1));
	}
	catch (Exception e) {
		notNumber = true;			
	}
	return notNumber;
}

Probably an overkill, but I try to avoid regex whenever I can.

To verify only first letter is number or character -- For number Character.isDigit(str.charAt(0)) --return true

For character Character.isLetter(str.charAt(0)) --return true