So far I have two issues with your solution:
1. It doesn't accept my email address. Looks like the pattern is way too strict and doesn't allow any new TLDs.
2. The use-case I most often see Splitwise used for is trips abroad with friends. In this situation, expenses are often recorded while no internet connection is available and synced later. I'm not sure if IOU supports this, I haven't tried it.
1. Can you send an email to iou@iou.ch so I can check you address and fix the bug? 2. We will be building mobile apps as soon as possible with this functionality.
Don't validate the address beyond looking for an '@' in the string; just send an email with a confirmation link to see if it actually works.
For reference, I didn't try any of the more obscure email address features, such as comments or quotation. My address simply has the form firstname@lastname.email
Please don't enable anyone to just send emails to arbitrary email-addresses through you. That's how your email/domain will quickly get marked for SPAM.
At the very least, put in place a rate-limit per email-id (and source-ip too maybe). Otherwise i can simply keep entering email-ids of people to spam.
^[^@]+@[^.]+\..+$