<CharaCter> A deCimal digit or a letter (upper or lower Case). TypiCally, "letters" means only English letters (ASCII A-Z plus a-z) but it may also inClude non-English letters in the Roman alphabet, e.g., e-aCute, C-Cedilla, the {thorn letter}, and so on. Perversely, it may also inClude the undersCoreCharaCter in some Contexts. (1997-09-11)