How to remove all non-alphanumeric characters from a string in Java

Given string str, the task is to remove all non-alphanumeric characters from it and print the modified it. 

Examples:

Input: @!Geeks-for'Geeks,123 
Output: GeeksforGeeks123 
Explanation: at symbol(@), exclamation point(!), dash(-), apostrophes('), 
and commas(, ) are removed.

Input: Geeks_for$ Geeks?{}[] 
Output: GeeksforGeeks 
Explanation: underscore(_), dollar sign($), white space, question mark(?), curly braces({}), 
and square bracket([]) are removed.

Input: GeeksforGeeks123 
Output: GeeksforGeeks123 
Explanation: No need to remove any character, because the 
given string doesn't have any non-alphanumeric character.

Method 1: Using ASCII values

Since the alphanumeric characters lie in the ASCII value range of [65, 90] for uppercase alphabets, [97, 122] for lowercase alphabets, and [48, 57] for digits. Hence traverse the string character by character and fetch the ASCII value of each character. If the ASCII value is not in the above three ranges, then the character is a non-alphanumeric character. Therefore skip such characters and add the rest in another string and print it.

Method 2: Using String.replaceAll()

Non-alphanumeric characters comprise of all the characters except alphabets and numbers. It can be punctuation characters like exclamation mark(!), at symbol(@), commas(, ), question mark(?), colon(:), dash(-) etc and special characters like dollar sign($), equal symbol(=), plus sign(+), apostrophes(‘). 

The approach is to use the String.replaceAll method to replace all the non-alphanumeric characters with an empty string.

Below is the implementation of the above approach: 

Java

// Java program to remove non-alphanumeric
// characters from a string
class GFG {

    // Function to remove non-alphanumeric
    // characters from string
    public static String
      removeNonAlphanumeric(String str)
    {
        // replace the given string
        // with empty string
        // except the pattern "[^a-zA-Z0-9]"
        str = str.replaceAll(
          "[^a-zA-Z0-9]", "");

        // return string
        return str;
    }

    // Driver Code
    public static void main(String args[])
    {

        // Test Case 1:
        String str1
          = "@!Geeks-for'Geeks, 123";
        System.out.println(
          removeNonAlphanumeric(str1));

        // Test Case 2:
        String str2
          = "Geeks_for$ Geeks?{}[]";
        System.out.println(
          removeNonAlphanumeric(str2));

        // Test Case 3:
        String str3
          = "GeeksforGeeks123";
        System.out.println(
          removeNonAlphanumeric(str3));
    }
}

Output

GeeksforGeeks123
GeeksforGeeks
GeeksforGeeks123

Method 3: Using Regular Expression

Another approach involved using of regular expression. The string can be easily filtered using the ReGex [^a-zA-Z0-9 ].

Java

// Java program to remove non-alphanumeric
// characters from a string

class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Input string
        String str1 = "@!Geeks-for'Geeks, 123";
        str1 = str1.replaceAll("[^a-zA-Z0-9]", "");

        System.out.println(str1);
    }
}

Output

GeeksforGeeks123

Method 4: Using isAlphabetic() and isDigit() methods

Java

// Java program to remove non-alphanumeric
// characters from a string

// Main class
class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Custom input string
        String str1 = "@!Geeks-for'Geeks, 123";
        String newstr = "";

        for (int i = 0; i < str1.length(); i++) {

            boolean b1 = Character.isDigit(str1.charAt(i));
            boolean b2 = Character.isAlphabetic(str1.charAt(i));

            if (b1 || b2) {
                newstr += str1.substring(i, i + 1);
            }
        }

        System.out.println(newstr);
    }
}

Output

GeeksforGeeks123