Lately I’ve been digging a bit into POSIX regular expressions. My first experience with regular exressions was in year 2000 with Perl. Since then, Perl-compatible regular expressions (PCRE) is the norm in higher level programming languages and new libraries. My encounter with POSIX regular expressions was entirely accidental, while examining the LLVM Regex utilities. These turned out to be based on OpenBSD’s implementation of POSIX regexes.