++I0! Minor changes to remove the start/stop markers which I think was desired:

# The golfish way:
    $text =~ s#(\s)\s+|STARTPRESERVE(.*?)STOPPRESERVE#$1$2#gs;
    # Update:  Oops!  This fails if a run
    # of whitespace doesn't start with ' '.

# or, if you don't like warnings:
    $text =~ s#\s{2,}|STARTPRESERVE(.*?)STOPPRESERVE#
      defined $1 ? $1 : " " #seg;
[download]

(:
        - tye (but my friends call me "Tye")

Greetings I0,

++! That's amazingly cool code. I love one-liners like that. In my specific situation, I don't need the START/STOP markers removed, and I can't imagine a senario where the data string would contain nested labels, however... I'm still curious. How would the nested labels version look?

(Note that I did spend some time trying to figure this out for myself so I could at least post a few bits of code here for others to revise, but I've failed to figure out even the basic theory without going into many, many lines of basic code.)

-gryphon
code('Perl') || die;

#Okay, you asked for it:

$_ = '0   1   
STARTPRESERVE
    STARTPRESERVE
        2 
    STOPPRESERVE
STOPPRESERVE
3   4
STARTPRESERVE
    STARTPRESERVE
        5
    STOPPRESERVE
STOPPRESERVE
6   7';

(my $re=$_)=~s/((STARTPRESERVE)|(STOPPRESERVE)|.)/${['(','']}[!$2]\Q$1
+\E${[')','']}[!$3]/gs;
$re= join'|',map{quotemeta}eval{/$re/};
die $@ if $@ =~ /unmatched/;
s/\s+|($re)/${[$1,' ']}[!$1]/g;
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$_ = "your long string";
my $output = "";
while (/\G(.*?)(STARTPRESERVE.*?STOPPRESERVE)/gcs) {
  my ($left, $right) = ($1, $2); # I wish I could shortcut that above 
+:(
  $left =~ s/\s+/ /g;
  $output .= "$left$right";
}
# last bit:
if (/\G(.+)/gcs) {
  my ($left) = $1;
  $left =~ s/\s+/ /g;
  $output .= $left;
}
[download]

-- Randal L. Schwartz, Perl hacker

And as another strategy:

my @temp = split /(STARTPRESERVE.*?STOPPRESERVE)/;
my $output = "";
while (@temp) {
  local $_ = shift @temp;
  s/\s+/ / unless @temp % 2;
  $output .= $_;
}
[download]

-- Randal L. Schwartz, Perl hacker

• Split based on the STARTPRESERVE and STOPPRESERVE markers, stuff the results into an array;
• Do the $text =~ s/\s+/ /g; on the first and last elements of the array; and
• Reassemble using join.

--t. alex

"Nyahhh (munch, munch) What's up, Doc?" --Bugs Bunny

use strict;

while(<DATA>) {
    my ($preserve) = $_ =~ /STARTPRESERVE(.*?)STOPPRESERVE/;
    s/$preserve// if $preserve;
    s/\s+/ /og;
    s/STARTPRESERVESTOPPRESERVE/$preserve/ if $preserve;
    print;
}

__DATA__
A Bridge Too Far     Hosted by Rod Stuart     Friendly Skys
42        STARTPRESERVE Life, the universe...    and      Everything  
+   STOPPRESERVE   X-Files
HotWheels are cool     More movies on Fox           File server
[download]

UPDATE UPDATE: that's it - i'm outta here ...

jeffa travels to the highest mountain to meditate more perlre

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

s[(STARTPRESERVE.*?STOPPRESERVE)|(\s+)|(.)]
 [($1||'') . ($2?' ':'') . ($3||'')]eg;
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Update: And this is of course wrong, since I'm testing with || and should do defined instead.

Update2: This is a correct and shorter version:

s#(STARTPRESERVE.*?STOPPRESERVE)|(\s+)|(.)#$1?$1:$2?' ':$3#eg;
[download]

Update3 (ouch):

s#(STARTPRESERVE.*?STOPPRESERVE)|(\s+)#$1?$1:$2?' ':''#eg;
[download]