This really ought to be table-driven so that custom tables can be loaded. For example, what about Unicode normalization? It's enforced on OS X. Maybe I want to

This really ought to be table-driven so that custom tables can be loaded. For example, what about Unicode normalization? It's enforced on OS X. Maybe I want to block anything that differs according to normalization form. I might want to limit characters to 2 accents at most. (this covers Vietnamese) I might want only left-to-right characters or LGC characters. I might want to block confusing look-alike characters, particularly those the non-ASCII ones that look like ASCII.