Java regex check alphanumeric string

By mkyong | Updated: November 8, 2020

Viewed: 19,770 (+191 pv/w)

Tags: alphanumeric java regex regex string regex

The Alphanumericals are a combination of alphabetical [a-zA-Z] and numerical [0-9] characters, 62 characters.

We can use below regex to match alphanumeric characters:

  ^[a-zA-Z0-9]+$

Regex explanation

  ^       # start string
  [a-z]   # lowercase letters from a to z
  [A-Z]   # uppercase letters from A to Z
  [0-9]   # digits from 0 to 9
  +       # one or more characters
  $       # end string

1. Java Regex Alphanumeric

Below is a Java regex to match alphanumeric characters.

package com.mkyong.regex.string;

public class StringUtils {

    private static final String ALPHANUMERIC_PATTERN = "^[a-zA-Z0-9]+$";

    public static boolean isAlphanumeric(final String input) {
        return input.matches(ALPHANUMERIC_PATTERN);
    }

}

Below is a simple JUnit 5 tests to validate the alphanumeric characters.

package com.mkyong.regex.string;

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.MethodSource;

import java.util.stream.Stream;

import static org.junit.jupiter.api.Assertions.assertFalse;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class StringUtilsTest {

    @ParameterizedTest(name = "#{index} - Run test with input = {0}")
    @MethodSource("validAlphanumericProvider")
    void test_alphanumeric_regex_valid(String input) {
        assertTrue(StringUtils.isAlphanumeric(input));
    }

    static Stream<String> validAlphanumericProvider() {
        return Stream.of(
                "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
                "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
                "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
                "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z",
                "0", "1", "2", "3", "4", "5", "6", "7", "8", "9");
    }

    @ParameterizedTest(name = "#{index} - Run test with input = {0}")
    @MethodSource("invalidAlphanumericProvider")
    void test_alphanumeric_regex_invalid(String input) {
        assertFalse(StringUtils.isAlphanumeric(input));
    }

    static Stream<String> invalidAlphanumericProvider() {
        return Stream.of(
                "_",                        // underscore
                "@",                        // symbols
                " ",                        // space
                "");                        // empty
    }

}

2. Regex Alphanumeric and Underscore

The regex \w is equivalent to [A-Za-z0-9_], matches alphanumeric characters and underscore.

package com.mkyong.regex.string;

public class StringUnderscore {

    public static void main(String[] args) {

        String str = "Hello_World_Java";

        // or ^[\\w]+$   
        if (str.matches("^[a-zA-Z0-9_]+$")) {
            System.out.println("Yes, true.");
        } else {
            System.out.println("failed!");
        }
    }
}

Output

  Yes, true.

3. Regex Alphanumeric and Space

The regex \\s matches whitespace characters.

package com.mkyong.regex.string;

public class StringSpace {

    public static void main(String[] args) {

        String str = "Hello World Java";
        if (str.matches("^[a-zA-Z0-9\\s]+$")) {
            System.out.println("Yes, true.");
        } else {
            System.out.println("failed!");
        }
    }
}

Output

  Yes, true.

Download Source Code

References

• Wikipedia – Alphanumericals
• Wikipedia – whitespace characters
• Java – How to remove spaces in between the String