Question

I have a telephone input field. You should only type numbers, spaces or leave it empty.
An idea how to build the regex?

Answer 1

The regex would be

^[\d ]*$      # in JavaScript, that's /^[\d ]*$/

but I would not do it this way. You should allow all characters, then filter out the numbers in a second step, using something like result = subject.replace(/\D+/g, "");.

Reason: People have all kinds of ways to enter phone numbers (1-[phone] etc.), and they don’t like it if a website tells them the number is incorrect.

Are you aware that you’re excluding country codes with this requirement? I have all my phone numbers stored as [phone], and I wouldn’t like my country code to be mistaken for an area code.