Re: Seeking clarification on possible bug in regex using \G and /gc

[choroba]

I'm not able to reproduce the behaviour you describe. Tried in 5.18.2 and 5.27.7, code:

#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

{
    local $_ = 'foo';
    say '1 Start' if /\G foo/gcx;
    say '1 Mid'   if /\G .*/gcx;
    say '1 End'   if /\G \z/gcx;
}

{
    local $_ = 'foo';
    say '2 Start' if /\G foo/gcx;
    say '2 Mid'   if /\G .+/gcx;
    say '2 End'   if /\G \z/gcx;
}

Output:

1 Start
1 Mid
2 Start
2 End

[davido]

Thanks choroba for setting me straight. If there's one thing that I keep forgetting to learn it's not to post regex questions or answers in haste. ;)

You are correct. This:

local $_ = 'foo';                         
                                          
m/\Gfoo/gc && say 'Matched foo';          
m/\G.+/gc  && say 'Matched dot star';     
m/\G\z/gc  && say 'Matched end of string';

...produces this:

Matched foo
Matched end of string

And this:

local $_ = 'foo';                         
                                          
m/\Gfoo/gc && say 'Matched foo';          
m/\G.*/gc  && say 'Matched dot star';     
m/\G\z/gc  && say 'Matched end of string';

...produces this:

Matched foo
Matched dot star

...indicating that end of string was consumed by dot star, though it's still a little odd because this also matches:

local $_ = 'foo';                      
                                       
m/\Gfoo/gc && say 'Matched foo';       
m/\G.*\z/gc  && say 'Matched dot star and end of string';

...produces this:

Matched foo
Matched dot star and end of string

So while it may be eluded to in the documentation it's not entirely unsurprising. :)

[Rhandom]

Breaks for me on perl 5.26.

perl -E 'local $_ = "foo"; say "Start" if /\G foo/gcx; say "Mid" if /\
+G .*/gcx; say "End" if /\G \z/gcx'
Start
Mid

Should instead print

Start
Mid
End

Here is a more extended item looking at pos as well.

perl -E 'local $_ = "foo"; say "Start" if /\G foo/gcx; say pos; say "M
+id" if /\G .*/gcx; say pos; say "End" if /\G \z/gcx; say "At End" if 
+pos == length'
Start
3
Mid
3
At End

[choroba]

> Should instead print

Why? Using perl -Mre=debug will show you your mistake:

...
Matching REx "\G \z" against ""
   3 <foo> <>                |  1:GPOS(2)
   3 <foo> <>                |  2:EOS(3)
   3 <foo> <>                |  3:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
Match failed

The length=1 request comes from the previously mentioned Repeated Patterns Matching a Zero-length Substring:

The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero.