Hello SuicideJunkie, and thanks for the answer. Unfortunately, I’m still confused. :-(

From your explanation, I would expect that making the whitespace match non-greedy would prevent the intermediate newline(s) from being eliminated. But it doesn’t (see below). Here is my current understanding (obviously flawed) of what should happen:

• ^ and $ are zero-width assertions, so when they feature in a match the newline they follow/preceed is not substituted. For example:

  18:14 >perl -wE "my $s = qq[\n\n\n]; my $t = $s =~ s{$}{}gmr; say $s e
  +q $t;"
  1
  
  18:14 >
  [download]

• \s*? matches zero or more whitespace characters (including newline) non-greedily.

• With the /g modifier in effect, whenever a match succeeds the regex engine begins looking for the next match one character past where the last successful match began.

Given these assumptions, I would expect that the regex /^\s*?$/ would match the string "a\n\n\nb" as follows: First, ^ matches after the first newline. Since \s*? is non-greedy, the regex engine looks for the shortest match satisfying \s*?$, and finds it in the zero-length string between the first two newlines. This it replaces with another zero-length string. It then starts looking for the next match with ^ matching after the second newline. Again, it finds and replaces a zero-length string. Finaly, ^ matches after the final newline, but no match is found. Result: the string is unchanged. However:

#! perl
use strict;
use warnings;

my $s = "a\n\n\nb";
my $t = $s =~ s{^\s*?$}{}gmr;

printf "%s\n", $s eq $t ? 'success' : 'fail';
print  ">$s<\n";
print  "[$t]\n";
[download]

18:29 >perl 902_SoPW.pl
fail
>a


b<
[a

b]

18:29 >
[download]

...
Guessed: match at offset 0
   2 <a%n> <%n%nb>           |  1:MBOL(2)
   2 <a%n> <%n%nb>           |  2:MINMOD(3)
   2 <a%n> <%n%nb>           |  3:STAR(5)
   2 <a%n> <%n%nb>           |  5:  MEOL(6)
   2 <a%n> <%n%nb>           |  6:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  POSIXD[\s] can match 1 times out of 
+1...
   3 <a%n%n> <%nb>           |  5:  MEOL(6)
   3 <a%n%n> <%nb>           |  6:  END(0)
Match successful!
...
[download]

What am I missing?