unicode_inverse(x, char_class = TRUE)

unicode_union(..., char_class = TRUE)

unicode_intersect(x, y, char_class = TRUE)

unicode_setdiff(x, y, char_class = TRUE)

# POSIX [:punct:] is more or less equivalent to the union of
# Unicode punctuation and symbol general categories
unicode_union(ugc_punctuation(), ugc_symbol())

# Everything except "A" to "Z" (including punctuation, control chars etc.)
unicode_inverse("[A-Z]")

# Uppercase letters, except "A" to "Z"
unicode_setdiff(ugc_uppercase_letter(), "[A-Z]")

# "A" to "F" (in upper or lower case)
unicode_intersect(ugc_letter(), up_ascii_hex_digit())

# Usage
x <- c(letters, LETTERS)
rx <- unicode_intersect(ugc_letter(), up_ascii_hex_digit())
stringi::stri_extract_first_regex(x, rx)