Resetting pos() within a RegEx?

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

the following example shows how to "ascii-draw" a tree with one regex!



  DB<83> $_="x"x30


  DB<84> 1 while s/^( *x+)xx/ $1\n$&/;print "$_\n $1\n $1"
              xx
             xxxx
            xxxxxx
           xxxxxxxx
          xxxxxxxxxx
         xxxxxxxxxxxx
        xxxxxxxxxxxxxx
       xxxxxxxxxxxxxxxx
      xxxxxxxxxxxxxxxxxx
     xxxxxxxxxxxxxxxxxxxx
    xxxxxxxxxxxxxxxxxxxxxx
   xxxxxxxxxxxxxxxxxxxxxxxx
  xxxxxxxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
              xx
              xx
[download]

That's for sure possible in a sole RegEx without while loop just with a /g-modifier!

But how? I experimented with \G but without success..

Cheers Rolf

PS: I know I missed Xmas... ; )

more golfing at Re: Christmas Tree

UPDATE: added 2 missing whitespaces

Comment on Resetting pos() within a RegEx? Download Code

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Re: Resetting pos() within a RegEx? by ikegami (Patriarch) on Jan 10, 2009 at 18:33 UTC
It's not possible to do in a s/// using the method you requested. If it was, the following would print a series of 'a's `>perl -le"$\|=1; ($_='abc') =~ s/\G(.)/print(pos(),' ',$1); pos()=0/eg" 0 a 1 b 2 c` [download]	[reply] [d/l]
Re: Resetting pos() within a RegEx? by Skeeve (Parson) on Jan 10, 2009 at 21:41 UTC
No loop but 2 regexps: $_="x" x 30; s:xx:(" " x (length($')/2))."$`$&\n":ge; s:(?<=\n).:$&$`$`:s; print; [download] `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re: Resetting pos() within a RegEx? by JavaFan (Canon) on Jan 11, 2009 at 01:06 UTC
That's for sure possible in a sole RegEx without while loop just with a /g-modifier! No. A regexp cannot print anything. Somewhere, you need to execute code, either outside a regexy construct, in the replacement part of an s/// with the /e modifier, or by using (?{ }) or (??{ }). But if I'm allowed to execute code, than the constraint "do it in a single regexp" isn't a constraint at all. Then I just do `/^(?{ code to generate whatever you want })/`. I suggest to be more specific in what you (dis)allow to have a more fun challenge.	[reply] [d/l]
Re^2: Resetting pos() within a RegEx? by LanX (Saint) on Jan 11, 2009 at 01:55 UTC
> I suggest to be more specific in what you (dis)allow to have a more fun challenge. It's for golfing, I just want to get shorter code by getting rid of the `while`-command not neccessarily replacing the print command.	[reply] [d/l]
Re^2: Resetting pos() within a RegEx? by Skeeve (Parson) on Jan 11, 2009 at 09:14 UTC
But if I'm allowed to execute code, than the constraint "do it in a single regexp" isn't a constraint at all. So here we go with 3 regexps and no code execution (exept for initializing and printing): $_="x" x 30; s:xx:$' $`$&\n:g; s:(?<=\n).:$&$`$`:s; s/xx(?=(x)(\1) )/ /g; print; [download] `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re^3: Resetting pos() within a RegEx? by LanX (Saint) on Jan 11, 2009 at 17:44 UTC
Still many bytes, but I really like your approach of taking the postmatch to indend the substitution! =) like this it's quick and easy to get the triangular part of the tree!!! DB<<70>> $_=$"x15;s: :$'#$`$`#\n:g;print ## # # # # # # # # # # # # # # # # # # # # # # # # # # # # DB<<71>> $_=$"x15;s: :$_=$`;y/ /#/;"$'$_$_\n":ge;print ## #### ###### ######## ########## ############ ############## ################ ################## #################### ###################### ######################## ########################## ############################ [download]	[reply] [d/l]
Re: Resetting pos() within a RegEx? by JavaFan (Canon) on Jan 12, 2009 at 14:02 UTC
Doing it in a single regexp is possible, although it won't be what you're looking at: `s{.*} { xx xxxx xxxxxx xxxxxxxx xxxxxxxxxx xxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xx xx}s;` [download]	[reply] [d/l]