Another puzzled RegEx Problem.

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";
[download]

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;
[download]

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.
[download]

TIA.
Pysome

Comment on Another puzzled RegEx Problem. Select or Download Code

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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;` [download] `1 while $d =~ s/(?<=\d\d)(?=\d)/-/;` [download] `$d = reverse($d); $d =~ s/(?<=\d)(?=(\d\d)+$)/-/g; $d = reverse($d);` [download] Update*: Added solutions.	[reply] [d/l] [select]
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";` [download]	[reply] [d/l] [select]
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 "-", $d =~ /(\d{1,2})/g;` [download]	[reply] [d/l]
Re^2: Another puzzled RegEx Problem. by pysome (Scribe) on Sep 13, 2007 at 00:59 UTC
Yes.	[reply]
Re: Another puzzled RegEx Problem. by shmem (Chancellor) on Sep 12, 2007 at 13:33 UTC
I wanna get a "right-to-left" substitution. See reverse. `$d = reverse "123456789"; $d =~ s/(?<=\d)(?=(\d\d)+$)/-/g; print scalar reverse "$d\n";` [download] update: added obvious solution --shmem _($_=" "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}	[reply] [d/l]
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;` [download] Update: Doh! Already posted above. Guess I should refresh more often!	[reply] [d/l]
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 $1 etc in substitution part --Artist	[reply] [d/l]
Re^2: Another puzzled RegEx Problem. by ikegami (Patriarch) on Sep 12, 2007 at 17:12 UTC
No external loop, no `reverse` and nothing but "`-`" in the substitution part: `$d =~ s/(?!^\|$)(?(?{ pos() % 2 })(?!))/-/g;` [download] Update: Bug fix. I originally forgot to prevent "`-`" from being placed at the end.	[reply] [d/l] [select]
Re^3: Another puzzled RegEx Problem. by pysome (Scribe) on Sep 13, 2007 at 01:11 UTC
cool	[reply]


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