In every application there is some kind of data input validation needed (with regular expressions). The only options we have for data validation in the SF are defaults and/or triggers. SQL support for regular expressions is quite limited. Adding data validations in the SF feels cumbersome to me.
Why don't we make use of HTML5 input validation with the pattern attribute on the input tag. We get full support of javascript regular expression and it's much less work to add data validations. The validation can also be blocking, so the user can't submit the form/data if the data is not in the correct format. The validation message displayed to the user is also way more intuitive then a pop-up message from a default or trigger:
