On Mon, 8 Jul 2019, ND via Pcre-dev wrote:
> And if we disregards Perl's bugs then it seems (*COMMIT) in Perl works in a
> following manner:
>
> 1. Backtracking can't move to the left of COMMIT (this is PCRE behaviour too)
> 2. If COMMIT occurs then no advance match to any other position of subject can
> happen. No matter there are any other backtracking control verbs occurs after
> COMMIT or COMMIT occurs in atomic group/negative lookaround etc (this is not
> implemented by PCRE)
There is also a difference in the way Perl handles repeated groups.
Consider
Perl 5.030000 Regular Expressions
/\A(?:.(*COMMIT))*c/
abcd
0: abc
PCRE2 version 10.34-RC1 2019-04-22
/\A(?:.(*COMMIT))*c/
abcd
No match
In Perl, the group repeat matches "abcd", but when it then does not
match "c", it unwinds complete repetitions of the group. In PCRE2,
there is a backtrack onto *COMMIT, so it fails. Looks like Perl handles
*COMMIT somehow differently to normal backtracks, because it does do
ordinary backtracks into repeated groups:
Perl 5.030000 Regular Expressions
/\A(.{1,2})*X/
AABBCX
0: AABBCX
1: C
Adding {1,2} to the first example gives this:
Perl 5.030000 Regular Expressions
/\A(?:.{1,2}(*COMMIT))*c/
abcd
No match
Having another backtrack point inside the group changes things, but then
I found this:
Perl 5.030000 Regular Expressions
/\A(?:(*COMMIT).)*c/
abcd
No match
I give up!
Philip
--
Philip Hazel
