[\d] = \d = [0-9]

Not exactly true. See perlrecharclass:

"\d" matches a single character that is considered to be a digit. What is considered a digit depends on the internal encoding of the source string and the locale that is in effect. If the source string is in UTF-8 format, "\d" not only matches the digits '0' - '9', but also Arabic, Devanagari and digits from other languages. Otherwise, if there is a locale in effect, it will match whatever characters the locale considers digits. Without a locale, "\d" matches the digits '0' to '9'. See "Locale, EBCDIC, Unicode and UTF-8".