(?P<NAME>...) vs (?<NAME>...)
Most Perl programmers using named captures in regex probably pick the
(?<NAME>...) syntax, as that's what's displayed more prominently in the Perl documentation and tutorials.
However, Python does not support this syntax and uses
(?P<NAME>...) instead (notice the extra P character). Incidentally, visual regex tool like kiki is built with Python and only support this syntax.
(?P<NAME>...) is also supported by Perl. So if you work with Python or use kiki, you might, like me, want to accustom yourself to using the P syntax.
PCRE (and thus PHP and other PCRE-using languages) supports both syntax. Komodo IDE's Rx toolkit support both. However, Ruby and .NET only support the non-P syntax. Well, that's how the real world works.