http://qs321.pair.com?node_id=638544

pysome has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks,
pls help me check this issues: E.g:
$d = "123456789";
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
print  "$d";

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It produce "1-23-45-67-89" ,it is OK.
But on the contrary,I wanna get a "left-to-right" substitution.That is ,i want to get "12-34-56-78-9"
I try :
$d =~ s/(?<=^(\d\d)+)(?=\d)/-/g;

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It can't work.Where does it wrong? The wrong log:
 
Variable length lookbehind not implemented in regex; marked by <-- HER
+E in m/(?<=^(\d\d)+)(?=\d) <-- HERE / at Noname1.pl line 4.

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TIA.
Pysome

Replies are listed 'Best First'.
Re: Another puzzled RegEx Problem.
by ikegami (Patriarch) on Sep 12, 2007 at 13:29 UTC

    Like the error message says, a limitation of (?<=re) and (?<!re) is that re must only match expressions of a known, fixed length. That means you can use ?, *, +, etc in re. Basically, the regexp engine needs to know how many characters to look back.

    Some alternatives: 

    $d =~ s/(\d\d)(?=\d)/$1-/g;

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    1 while $d =~ s/(?<=\d\d)(?=\d)/-/;

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    $d = reverse($d);
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
$d = reverse($d);

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    Update: Added solutions.

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Re: Another puzzled RegEx Problem.
by Fletch (Bishop) on Sep 12, 2007 at 13:32 UTC

    Lookbehind has to be a fixed width; yours isn't (the + quantifier) so it's an invalid regex.

    This is a little kludgy, but will work: 

    my $str = "123456789";
$str = reverse $str;
$str =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
$str = reverse $str;
print $str, "\n";

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Re: Another puzzled RegEx Problem.
by Prof Vince (Friar) on Sep 12, 2007 at 16:53 UTC
    Substitutions aren't everything. How about :
    my $d = "123456789";
$d = join "-", $d =~ /(\d{1,2})/g;

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      Yes.
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Re: Another puzzled RegEx Problem.
by shmem (Chancellor) on Sep 12, 2007 at 13:33 UTC
    I wanna get a "right-to-left" substitution.
    See reverse. 
    $d = reverse "123456789";
$d =~ s/(?<=\d)(?=(\d\d)+$)/-/g;
print scalar reverse "$d\n";

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    update: added obvious solution

    --shmem 

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
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Re: Another puzzled RegEx Problem.
by johnlawrence (Monk) on Sep 12, 2007 at 13:59 UTC
    This should also work, though I did like the previous solution of using reverse. 
    $d =~ s/(\d\d)(?=\d)/$1-/g;

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    Update: Doh! Already posted above. Guess I should refresh more often!
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Re: Another puzzled RegEx Problem.
by artist (Parson) on Sep 12, 2007 at 15:11 UTC
    No solution here, but the challenge would be to find the solution that has
    • No while
    • No reverse
    • No $1 etc in substitution part
    --Artist
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      No external loop, no reverse and nothing but "-" in the substitution part: 

      $d =~ s/(?!^|$)(?(?{ pos() % 2 })(?!))/-/g;

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      Update: Bug fix. I originally forgot to prevent "-" from being placed at the end.

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        cool
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