[Python-Dev] Why Foo is better than Baz

[Tim Peters]

[Guido & Andrew on Tcl's new regexp code]
> I'm sure that if it's good code, we'll find a way.  Perhaps a more
> interesting question is whether it is Perl5 compatible.  I contacted
> Henry Spencer at the time and he was willing to let us use his code.

Haven't looked at the code, but did read the manpage just now:

    http://www.scriptics.com/man/tcl8.1/TclCmd/regexp.htm

WRT Perl5 compatibility, it sez:

    Incompatibilities of note include `\b', `\B', the lack of special
    treatment for a trailing newline, the addition of complemented
    bracket expressions to the things affected by newline-sensitive
    matching, the restrictions on parentheses and back references in
    lookahead constraints, and the longest/shortest-match (rather than
    first-match) matching semantics.

So some gratuitous differences, and maybe a killer:  Guido hasn't had much
kind to say about "longest" (aka POSIX) matching semantics.  An example from
the page:

    (week|wee)(night|knights)
    matches all ten characters of `weeknights'

which means it matched 'wee' and 'knights'; Python/Perl match 'week' and
'night'.

It's the *natural* semantics if Andrew's suspicion that it's compiling a DFA
is correct; indeed, it's a pain to get that behavior any other way!

otoh-it's-potentially-very-much-faster-ly y'rs  - tim