This is true for many regular expression dialects. But this bug about POSIX regular expressions. POSIX doesn't define \d, and it defines ranges such as [0-9] o

This is true for many regular expression dialects. But this bug about POSIX regular expressions. POSIX doesn't define \d, and it defines ranges such as [0-9] only in the POSIX locale. Historically, glibc considered such ranges in the context of locale's collation order. For example, in Estonian locale, [a-z] doesn't include the letters tuvwxy, because that's how Estonian alphabet works: https://en.wikipedia.org/wiki/Estonian_orthography#Alphabet https://en.wikipedia.org/wiki/Estonian_orthography#Alphabet This (somewhat surprising) semantics of character ranges is not new, but the recent changes in glibc made it more spectacular.