Determine if a string starts with a number in Java

This post will check if a string starts with a number or not in Java.

1. Naive solution

A naive solution is to extract the first character from the given string using the charAt() method and validate that character falls under the numeric ASCII range or not.

class Main

{

public static void main(String[] args)

{

String str = "12ABC";

char ch = str.charAt(0);

System.out.println(ch >= '0' && ch <= '9'); // true

}

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2. Using `Character.isDigit()` method

The recommended solution is to use the Character.isDigit(char) method to check if the first character of the string is a digit or not. Note that this also supports all Unicode digits.

class Main

{

public static void main(String[] args)

{

String str = "12ABC";

System.out.println(Character.isDigit(str.charAt(0))); // true

}

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3. Using Regex

Another plausible way of determining whether a string ends with a number or not is using regex. This approach is not recommended as regular expressions are extremely slow.

class Main

{

public static void main(String[] args)

{

String str = "12ABC";

System.out.println(str.matches("\\d.*")); // true

System.out.println(str.matches("[0-9].*")); // true

}

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Note that all the above solutions assume that the string is not empty. If the string is empty, the program will throw a StringIndexOutOfBoundsException.

That’s all about checking if a string starts with a number in Java.

Determine if a string starts with a number in Java

1. Naive solution

2. Using Character.isDigit() method

3. Using Regex

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2. Using `Character.isDigit()` method