Transition to using regex

[cyrusae]

Per this 2022 post, base grepl with perl = TRUE is the best-looking option and avoids novel dependencies.

The mixed-case single-character variant is the first priority, then everything else is case-agnostic and can be transformed once to use a single reference table.

Filter on [:alpha:] to avoid chaff in input.

[cyrusae]

EXCEPT FOR @. MIXED-CASE NEEDS TO RETAIN @ BEFORE MOVING ON DO NOT FORGET THIS

[cyrusae]

i.e. [[:alpha:]@#] initial filter, since @ is a legal symbol and H# needs to be NA'd later