Exim is an MTA (mail transfer agent) that is intended to be a drop-in replacement for Sendmail. It was originally written in 1995 by Philip Hazel.
In 1997 he replaced its POSIX regular expression library written by Henry Spencer with a new library Hazel developed called PCRE (Perl Compatible Regular Expressions). Perl regular expressions are much more powerful than POSIX and other common regular expressions, so PCRE has become popular in many applications other than Exim.
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