如何递归一个字符串以验证它仅包含第一个字符之后的数字

Question

I need to create a recursive method that takes a String parameter.
I need to verify that the first letter in the string is a lowercase character, and then verify that everything that comes after the first letter is a number.

So it should be like this a1234 .

I've tried creating a base case, smaller base case, and a general case, but can't seem to figure out the right way to style it:

public void digitCheck(String s) {

   if () //To Check that first character is a letter

   else if ()To check that everything after the first letter is a number

   else //Invalid
}

I need the code to report whether it's a valid string if the first character is a lower case letter and everything after that is a number .

Example:
a123 -> valid .
ab123 -> invalid .

Answer 1

使用String.matches（）方法：

boolean valid = s.matches(".\\d+");

Answer 2

For solving this problem with recursion for your pattern you can do:

• start from the end and
• check the last element
• remove the last element
• call all remaining part

It should be checking until one element will be passed to the method -> make final validation if it is a lower letter.

Also, StringUtils class is used from commons lang library.

Here is code snippet:

public class StringValidationDemo {
    public boolean validateStringRecursive(String str) {
        if (str.length() == 1) {
            return StringUtils.isAlpha(str) && StringUtils.isAllLowerCase(str);
        }

        String lastIndex = str.substring(str.length() - 1);
        return StringUtils.isNumeric(lastIndex)
                && validateStringRecursive(str.substring(0, str.length() - 1));
    }

    public static void main(String[] args) {
        List<String> words = Arrays.asList("a123", "ab123", "123ab", "A123", "aaa", "123");

        StringValidationDemo demo = new StringValidationDemo();
        for (String word : words) {
            System.out.printf("validation for: %s = %s\n",
                    word, demo.validateStringRecursive(word));
        }
    }
}

Output:

validation for: a123 = true
validation for: ab123 = false
validation for: 123ab = false
validation for: A123 = false
validation for: aaa = false
validation for: 123 = false

Answer 3

I think you could ommit the first character in the string, and then just check with Integer.parseInt(String). so it would look something like:

public static boolean isNumeric(String strNum) {
    try {
        double d = Integer.parseInt(String);
    } catch (NumberFormatException | NullPointerException nfe) {
        return false;
    }
    return true;
}

public void DoStuff(String string){
    if (isNumeratic(string.substring(1)) //ommits first
    {
      ///yourstuff
    }
}