How to Check if a String Contains Uppercase and Lowercase in JavaScript

Checking if a string contains both uppercase and lowercase letters is a common requirement in tasks like password validation and input verification. JavaScript provides several methods to accomplish this.

function containsUppercaseAndLowercase(str) {
    return /[A-Z]/.test(str) && /[a-z]/.test(str);
}

let str = "HelloWorld";
let result = containsUppercaseAndLowercase(str);
console.log(result); // true

To check if a string contains both uppercase and lowercase letters in JavaScript, use regular expressions:

In many scenarios, especially in user input validation, it’s crucial to check whether a string contains both uppercase and lowercase letters. This guide explores how to perform this check efficiently using JavaScript.

Using Regular Expressions

Regular expressions (regex) provide a powerful way to search for patterns in strings. You can use regex to check for both uppercase and lowercase letters.

Example 1: Basic Regex Check

let str = "HelloWorld";
let hasUppercase = /[A-Z]/.test(str);
let hasLowercase = /[a-z]/.test(str);

let result = hasUppercase && hasLowercase;
console.log(result); // true

Explanation:

• /[A-Z]/: Checks if the string contains any uppercase letters.
• /[a-z]/: Checks if the string contains any lowercase letters.
• test(str): Returns true if the pattern is found in the string.
• result: Combines both checks to determine if the string contains both uppercase and lowercase letters.

Using a Custom Function

Encapsulating the logic in a custom function makes it reusable and more maintainable.

Example 2: Custom Function

function containsUppercaseAndLowercase(str) {
    return /[A-Z]/.test(str) && /[a-z]/.test(str);
}

let str1 = "HelloWorld";
let str2 = "helloworld";
let str3 = "HELLOWORLD";

console.log(containsUppercaseAndLowercase(str1)); // true
console.log(containsUppercaseAndLowercase(str2)); // false
console.log(containsUppercaseAndLowercase(str3)); // false

Explanation:

• containsUppercaseAndLowercase(str): A function that checks if the string contains both uppercase and lowercase letters.
• Return value: The function returns true if both uppercase and lowercase letters are present, otherwise false.

Example Scenarios

Example 3: Complex Strings

let str = "123abcABC";
console.log(containsUppercaseAndLowercase(str)); // true

Explanation:

• Mixed content: The function correctly identifies both uppercase and lowercase letters even when the string contains numbers or other characters.

Example 4: No Uppercase Letters

let str = "javascript";
console.log(containsUppercaseAndLowercase(str)); // false

Explanation:

• Lowercase only: The function returns false since there are no uppercase letters.

Example 5: No Lowercase Letters

let str = "JAVASCRIPT";
console.log(containsUppercaseAndLowercase(str)); // false

Explanation:

• Uppercase only: The function returns false since there are no lowercase letters.

Conclusion

Checking if a string contains both uppercase and lowercase letters in JavaScript can be efficiently done using regular expressions. By creating a custom function, you can encapsulate this logic and easily reuse it in various parts of your application. This method is particularly useful for tasks like password validation, where such checks are often required.