What is the best way to communicate to a User which characters are allowable in a Form?
If space wasn't an issue, then you could write a book to eliminate ambiguity. However that is rarely the case!
Here are some different Error messages all trying to susinctly communicate the same thing...
The last one could take them to a page with LARGE FONT spelling things out.$errors['username'] = 'Username must be 8-30 characters (A-Z0-9_- )';
$errors['username'] = 'Username can only use Letters, Numbers, Underscore, Hyphen, Period.';
$errors['username'] = 'Username can only use: a-z A-Z 0-9 _ - .';
$errors['username'] = 'Invalid Username. (see valid characters)';
As a developer, I think #1 is the best, however it may not be intuitive to your average person.
As Jane User, #2 is the clearest, but it also is really l--o--n--g, and if you had 8-10 special characters you allowed, then you would have wrapping issues!!
Anyways, what do you think?!