code review 152131: move evaluation of null-matching instructions one itera...

move evaluation of null-matching instructions one iteration earlier.
performance hit of about 20% but more intuitive results for submatches.
we need a good regexp package at some point.

Fixes issue 110.

[r, 2009-11-13 23:41:21]

Hello rsc (cc: golang-dev@googlegroups.com),

I'd like you to review the following change.

[rsc, 2009-11-13 23:48:57]

LGTM

http://codereview.appspot.com/152131/diff/1/3
File src/pkg/regexp/regexp.go (right):

http://codereview.appspot.com/152131/diff/1/3#newcode657
src/pkg/regexp/regexp.go:657: s = re.addState(s, inst.next(), match, pos, end);
This sequence should probably be

old := match[2*n];
match[2*n] = pos;
s = re.addState(s, inst.next(), match, pos, end);
match[2*n] = old;

so that the caller's match doesn't get edited.
Same in _EBRA.  That requires copying match
below (marked).

http://codereview.appspot.com/152131/diff/1/3#newcode685
src/pkg/regexp/regexp.go:685: s[l].match = match;
I suggest putting the copy here of match here.
Then you can get rid of the two copies in _ALT below.

[r, 2009-11-14 20:11:39]

http://codereview.appspot.com/152131/diff/1/3
File src/pkg/regexp/regexp.go (right):

http://codereview.appspot.com/152131/diff/1/3#newcode657
src/pkg/regexp/regexp.go:657: s = re.addState(s, inst.next(), match, pos, end);
i tried these changes; regex-dna slowed by 50%.

[r, 2009-11-14 20:23:25]

*** Submitted as http://code.google.com/p/go/source/detail?r=21cae7efdcc6 ***

move evaluation of null-matching instructions one iteration earlier.
performance hit of about 20% but more intuitive results for submatches.
we need a good regexp package at some point.

Fixes issue 110.

R=rsc
CC=golang-dev
http://codereview.appspot.com/152131