Hello choroba,

Thanks for the explanation, and I’m sorry to be obtuse but — I still don’t understand. :-( Consider:

#! perl -l
use strict;
use warnings;

my $s = 'uvXYZdabcXYZfg';

while ($s =~ /(\w\w\w)*?(XYZ)/g)
{
    print 'Found match ', $1, $2, ' at pos: ', pos $s;
}

print '-----';

while ($s =~ /(abc)*?(XYZ)/g)
{
    print 'Found match ', $1, $2, ' at pos: ', pos $s;
}
[download]

Output:

22:35 >perl 1476_SoPW.pl
Found match abcXYZ at pos: 12
-----
Use of uninitialized value $1 in print at 1476_SoPW.pl line 28.
Found match XYZ at pos: 5
Found match abcXYZ at pos: 12

22:35 >
[download]

The first capture in each regex is 3 characters wide, but the first regex matches only the second occurrence of XYZ whereas the second regex also matches the first occurrence, with (abc)*? matching zero times. Why the difference in behaviour? In particular, why does (\w\w\w)*? not also match zero times?

In the first case, the engine starts from the left and finds a match, repeating (\w\w\w) 3 times:

uvXYZdabcXYZfg
^        ^   
|        |
A        B
[download]

The engine than starts to match at B + 1, and finds no such a match.

In the second case, the engine starts from the left as well, but finds no match:

uvXYZdabcXYZfg
^
|
A
[download]

So, it moves to A + 1 (still no match), and then A + 2, where it can match with (abc)* repeating zero times:

uvXYZdabcXYZfg
  ^
  |
 A=B
[download]

After matching, it continues (because of /g) to B + 3 (no match), and at B + 4 it finally succeeds with

uvXYZdabcXYZfg
      ^  ^
      |  |
      A  B
[download]

Better now?

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Yes, thanks, I’m starting to see some light. :-) But:

In the first case, the engine starts from the left and finds a match, repeating (\w\w\w) 3 times:

Makes sense, but then, if the $1 match effectively ends up as (\w\w\w){3}, shouldn’t $1 contain uvXYZdabc? Or, conversely, if the ? quantifier following the * causes it to find the minimum string in uvXYZdabc matching (\w\w\w)*, shouldn’t that be "", the null string? Why does $1 match abc here?

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,