PCRE stands for "Perl Compatible Regular Expressions". Which used to implement a superset of Perl regular expression, but we have since (5.10) caught up.

But the OP asked for POSIX regular expressions. Which, except for syntax differences, actually behave differently:

  "abc" =~ /.|../ && print $&;
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Prints 'a' with Perl, but a POSIX style regexp engine would result in 'ab' being printed. With POSIX style, "longest" trumps "leftmost".