Check if a JavaScript string contains only alpha or alphanumeric characters

Oftentimes, strings are used to represent data that should only contain alphabetic or alphanumeric characters. Using regular expressions, you can easily check if a string matches the pattern.

Check if a string contains only alpha characters

For alpha (alphabetic) characters, you can use a range ([a-zA-Z]) to match any character from a to z (lowercase) and A to Z (uppercase). Simply adding the * quantifier and the positional anchors (^ and $) will ensure that the pattern matches the entire string. Then, you can use RegExp.prototype.test() to check if the given string matches against the alphabetic pattern.

const isAlpha = str => /^[a-zA-Z]*$/.test(str);

isAlpha('sampleInput'); // true
isAlpha('this Will fail'); // false
isAlpha('123'); // false

Check if a string contains only alphanumeric characters

Subsequently, you can extend the pattern to include digits (0-9) by adding them to the range. The resulting pattern will be [a-zA-Z0-9]. Adding the gi flags makes the pattern case-insensitive, allowing for the range to be simplified to [a-z0-9].

const isAlphaNumeric = str => /^[a-z0-9]*$/gi.test(str);

isAlphaNumeric('hello123'); // true
isAlphaNumeric('123'); // true
isAlphaNumeric('hello 123'); // false (space character is not alphanumeric)
isAlphaNumeric('#$hello'); // false

[!TIP]

These methods can serve as a great starting point for more complex string validation patterns. You can further customize the regular expressions to suit your specific requirements (e.g. allowing spaces, hyphens, or underscores in the string).