Status report: regex replacement

I have just committed the latest version of Henry Spencer's regex
package (lifted from Tcl 8.4.1) into CVS HEAD. This code is natively
able to handle wide characters efficiently, and so it avoids the
multibyte performance problems recently exhibited by Wade Klaver.
I have not done extensive performance testing, but the new code seems
at least as fast as the old, and much faster in some cases.

Also, we now have a regex flavor that is an exact match for recent
Tcl releases and a close match for recent Perl releases; it sports
back references and lookahead among other niceties.

There's some stuff still to do:

1. There are a couple of minor incompatibilities between the "advanced"
regex syntax implemented by this package and the syntax handled by our
old code; in particular, backslash is now a special character within
bracket expressions. It seems to me that we'd better offer a switch
to allow backwards compatibility. This is easily done as far as the
code is concerned: the regex library actually offers three regex
flavors, "advanced", "extended", and "basic", where "extended" matches
what we had before ("extended" and "basic" correspond to different
levels of the POSIX 1003.2 standard). We just need a way to expose
that knob to the user. I am thinking about inventing yet another GUC
parameter, say

set regex_flavor = advanced
set regex_flavor = extended
set regex_flavor = basic

We could satisfy the immediate need with just a boolean "advanced_regex
= on/off", but it seems forward-looking to allow for the possibility of
more flavors in future. (For one thing, this would offer an easy place
to select a different regex package, in case anyone still wants to play
around with sre or the other alternatives that were mentioned
yesterday.)

Any suggestions about the name of the parameter?

2. Documentation. I've transformed Spencer's manual page into SGML and
added it to func.sgml, but it's starting to look a tad, um, bulky:
http://developer.postgresql.org/docs/postgres/functions-matching.html#FUNCTIONS-POSIX-REGEXP
The regex section now accounts for 1200+ out of func.sgml's 7500 lines.
Should it be split out as an appendix, or is it okay where it is?

3. I've been toying with the idea of getting rid of the special-purpose
matching code for LIKE (see adt/like.c and like_match.c), and
reimplementing LIKE as a front-end to the regex engine; all it would
need is to translate LIKE patterns into regex style, much as we already
do for SQL99's SIMILAR TO patterns. This would reduce the maintenance
needs for LIKE by a great deal. In some preliminary tests here, it
seemed that the special-purpose LIKE code is faster than equivalent
regex matching would be --- but I didn't try the multibyte code path,
nor any but the simplest of patterns. Anyone want to try some more
extensive benchmarking?

4. The new regex code is 8-bit-clean (no dependency on null-terminated
strings), so it'd be feasible to implement regex matching for BYTEA.
Over to you on that one, Joe.

regards, tom lane