TIL Regex for Decimal Points

I needed a regular expression for checking if a number has 1 decimal point. Here’s the regular expression ChatGPT came up with: /^\d+(\.\d{1})?$/

Of course, it wasn’t actually the correct requirement. It needed to check if it’s an integer (no decimal points). And if it is a decimal point, it can be at most one.

I screwed up and tried to pair two quantifiers together. So if you have ? and you want to use {0,1}, you can’t do {0.1}?.

And I didn’ need the group, although in retrospect, using the quantifier on the group works, but removing the group makes the ? obsolete.

Here’s what I ultimately came up with.

function validateToOneOrNoDecimalPoint(number) {
    // Regular expression to match numbers with either 0 or 1 decimal point (optional)
    const regex = /^\d+\.?\d{0,1}$/;
    return regex.test(number);
}

// Example usage:
console.log(validateToOneOrNoDecimalPoint(3.5));   // true
console.log(validateToOneOrNoDecimalPoint(10));    // true
console.log(validateToOneOrNoDecimalPoint(2.34));  // false (more than one decimal point)
console.log(validateToOneOrNoDecimalPoint(5.));    // true (no decimal point)
console.log(validateToOneOrNoDecimalPoint(5));     // true (no decimal point)

And I added even more test cases, and used regex.match over regex.test. Subtle difference.