If you've ever looked up "phone number regex" and regretted it, you're in the right place. There are a lot of valid phone number formats, but fortunately there are free tools that you can use to help make sure a phone number is valid. This post will walk through two ways to check a phone number's validity: the Twilio Lookup API and the intl-tel-input JavaScript plugin. This builds on How to build international phone number input in HTML and JavaScript, which you can reference for more details o...