Front-end registration validation
You could use Regex (Regular Expression), /^[a-zA-Z]{3,}$/ would for instance allow a minimum of 3 characters behind the comma would be the max value.

As for numbers it would be something like this /^[0-9]{3,}$/

http://regexr.com/ to test your regex en examples

And I would do this before the user is saved with the users.onBeforeSaveUser Event in a plugin.