Interesting. I just noticed (|f+) behaves the same.

Running a simplified code with use re 'debug' gives the answer: 

use re 'debug';
$s = 'xfx';
while ($s =~ / (f*?) /gx) {

[download]

Compiling REx " (f*?) "
Final program:
   1: OPEN1 (3)
   3:   MINMOD (4)
   4:   STAR (7)
   5:     EXACT <f> (0)
   7: CLOSE1 (9)
   9: END (0)
minlen 0 
xfx
------------
Matching REx " (f*?) " against "xfx"
   0 <> <xfx>                |  1:OPEN1(3)
   0 <> <xfx>                |  3:MINMOD(4)
   0 <> <xfx>                |  4:STAR(7)
   0 <> <xfx>                |  7:  CLOSE1(9)
   0 <> <xfx>                |  9:  END(0)
Match successful!
matched (f*?) of "", starting at offset of 0, ending at pos of 0 
Matching REx " (f*?) " against "xfx"
   0 <> <xfx>                |  1:OPEN1(3)
   0 <> <xfx>                |  3:MINMOD(4)
   0 <> <xfx>                |  4:STAR(7)
   0 <> <xfx>                |  7:  CLOSE1(9)
   0 <> <xfx>                |  9:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  EXACT <f> can match 0 times out of 1
+...
                                  failed...
   1 <x> <fx>                |  1:OPEN1(3)
   1 <x> <fx>                |  3:MINMOD(4)
   1 <x> <fx>                |  4:STAR(7)
   1 <x> <fx>                |  7:  CLOSE1(9)
   1 <x> <fx>                |  9:  END(0)
Match successful!
matched (f*?) of "", starting at offset of 1, ending at pos of 1 
Matching REx " (f*?) " against "fx"
   1 <x> <fx>                |  1:OPEN1(3)
   1 <x> <fx>                |  3:MINMOD(4)
   1 <x> <fx>                |  4:STAR(7)
   1 <x> <fx>                |  7:  CLOSE1(9)
   1 <x> <fx>                |  9:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  EXACT <f> can match 1 times out of 1
+...
   2 <xf> <x>                |  7:  CLOSE1(9)
   2 <xf> <x>                |  9:  END(0)
Match successful!
matched (f*?) of "f", starting at offset of 1, ending at pos of 2 
Matching REx " (f*?) " against "x"
   2 <xf> <x>                |  1:OPEN1(3)
   2 <xf> <x>                |  3:MINMOD(4)
   2 <xf> <x>                |  4:STAR(7)
   2 <xf> <x>                |  7:  CLOSE1(9)
   2 <xf> <x>                |  9:  END(0)
Match successful!
matched (f*?) of "", starting at offset of 2, ending at pos of 2 
Matching REx " (f*?) " against "x"
   2 <xf> <x>                |  1:OPEN1(3)
   2 <xf> <x>                |  3:MINMOD(4)
   2 <xf> <x>                |  4:STAR(7)
   2 <xf> <x>                |  7:  CLOSE1(9)
   2 <xf> <x>                |  9:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  EXACT <f> can match 0 times out of 1
+...
                                  failed...
   3 <xfx> <>                |  1:OPEN1(3)
   3 <xfx> <>                |  3:MINMOD(4)
   3 <xfx> <>                |  4:STAR(7)
   3 <xfx> <>                |  7:  CLOSE1(9)
   3 <xfx> <>                |  9:  END(0)
Match successful!
matched (f*?) of "", starting at offset of 3, ending at pos of 3 
Matching REx " (f*?) " against ""
   3 <xfx> <>                |  1:OPEN1(3)
   3 <xfx> <>                |  3:MINMOD(4)
   3 <xfx> <>                |  4:STAR(7)
   3 <xfx> <>                |  7:  CLOSE1(9)
   3 <xfx> <>                |  9:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  EXACT <f> can match 0 times out of 1
+...
                                  failed...
Match failed
Freeing REx: " (f*?) "

[download]

Do you see? 

Match possible, but length=0 is smaller than requested=1, failing!

[download]
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re^9: Understanding a portion of perlretut by choroba
in thread Understanding a portion on the Perlretut by BlueStarry

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.