Function is_ruby_word_char 

pub fn is_ruby_word_char(ch: char) -> bool

Onigmo’s UTF-8 word-char definition (what Ruby \b and \B consult — NOT \w, which is ASCII-only): the Unicode Word property = Alphabetic (L* + Nl) | Mark (Mn/Mc/Me) | Nd | Pc, PLUS the explicit ZWNJ/ZWJ additions (U+200C, U+200D) Onigmo special-cases — verified empirically against Ruby (a letter+ZWNJ pair is NOT a boundary). std exposes no general categories, hence unicode-general-category.