How to Create a Regular Expression to Accept Only Alphanumeric Characters

How can I write a regex pattern that matches only alphanumeric characters?

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

Regular expressions (regex) are powerful tools used for pattern matching and validation in strings. To create a regex that matches only alphanumeric characters (letters and numbers), we can utilize specific regex syntax. This guide will walk you through creating such a regex and provide insights into common issues and resolutions.

// Example in JavaScript
const alphanumericRegex = /^[a-zA-Z0-9]*$/;

const testString1 = "Hello123";
const testString2 = "Hello_123";

console.log(alphanumericRegex.test(testString1)); // true
console.log(alphanumericRegex.test(testString2)); // false (contains an underscore)

Causes

• Misunderstanding regex syntax
• Using the wrong anchors or character classes
• Not escaping special characters when necessary

Solutions

• Use the regex pattern "^[a-zA-Z0-9]*$" to match only alphanumeric characters including uppercase and lowercase letters.
• Ensure to specify boundaries using anchors (^) for the start and ($) for the end of the string to prevent matches with additional characters.

Mistake: Forgetting to include both uppercase and lowercase letters in the character class.

Solution: Ensure the regex includes both a-z and A-Z to cover all letter cases.

Mistake: Using a character class that is too broad, leading to unintended matches.

Solution: Stick to the predefined range of a-z0-9 to restrict matches to only alphanumeric characters.

Mistake: Not using anchors, resulting in partial matches.

Solution: Use '^' at the beginning and '$' at the end of your regex pattern.