Remove all characters from string except alphabets in Java

Understanding String Filtering

Filtering a string means removing all non-alphabetic characters from it.

We will explore three different methods to achieve this in Java.

Method 1: Using a Loop

This method iterates through the string and keeps only alphabetic characters.

public class RemoveNonAlpha {
    public static String removeNonAlpha(String str) {
        StringBuilder result = new StringBuilder();
        for (char c : str.toCharArray()) {
            if (Character.isLetter(c)) {
                result.append(c);
            }
        }
        return result.toString();
    }
    
    public static void main(String[] args) {
        String str = "H3ll0 W@rld!";
        System.out.println("Filtered String: " + removeNonAlpha(str));
    }
}
            

Method 2: Using Recursion

This method filters the string recursively.

public class RemoveNonAlphaRecursive {
    public static String filterRecursive(String str, int index) {
        if (index == str.length()) return "";
        char current = str.charAt(index);
        return (Character.isLetter(current) ? current : "") + filterRecursive(str, index + 1);
    }
    
    public static void main(String[] args) {
        String str = "C0d!ng123";
        System.out.println("Filtered String: " + filterRecursive(str, 0));
    }
}
            

Method 3: Using Regular Expressions

This method uses regex to replace all non-alphabetic characters.

public class RemoveNonAlphaRegex {
    public static String removeWithRegex(String str) {
        return str.replaceAll("[^a-zA-Z]", "");
    }
    
    public static void main(String[] args) {
        String str = "P@r0gr@mm1ng";
        System.out.println("Filtered String: " + removeWithRegex(str));
    }
}