Hey... how would you validate a full_name field (name surname).
Any validation you perform here is likely to break down unless it is extremely general. For instance, enforcing a minimum length of 3 is probably about as reasonable as you can get without getting into the specifics of what is entered.
When you have names like "O'Malley" with an apostrophe, "Smith-Johnson" with a dash, "Andrés" with accented characters or extremely short names such as "Vo Ly" with virtually no characters at all, how do you validate without excluding legitimate cases? It's not easy.