Excellent point, thank you for spotting that! I can confirm that the (?= ) around (?<lookback> ) can be removed in all cases in the root node (since (?<= ) is already zero-width). Makes the regexes even shorter! :-)

It's probably a vestige from the negative case like here or in the following, where the (?! (?<lookback> ... ) ) is needed*.

# Match any /\d./ that is *not* preceded by an /a/

my $re5 = qr{
    (?! (?<lookback>
        (?<=
            a
        |
            (?=(?&lookback)) .
        )
    ) )
    (?<target> \d . )
}msx;
my $re5_short = qr
    /(?!((?<= a |(?=(?-1)).))) (\d.) /sx;

for my $regex ($re5,$re5_short) {
    unlike "fo",        $regex;
    unlike "x5",        $regex;
    unlike "ab5 x4",    $regex;
      like "5ab",       $regex;
      like "x5 ab5",    $regex;
      like "x5 ab5 x2", $regex;
    my @results;
    while ("x2 4x3a55aaa1" =~ /$regex/g)
        { push @results, $+{target} // $2 }
    is_deeply \@results, ["2 ","4x","3a"];
}

[download]

* Update: Hmm, actually, it turns out this seems to work too... (although putting the exact explanation of why into words is eluding me at the moment...)

my $re5 = qr{
    (?<lookback>
        (?<!
            a
        |
            (?!(?&lookback)) .
        )
    )
    (?<target> \d . )
}msx;
my $re5_short = qr
    /((?<! a |(?!(?-1)).)) (\d.) /sx;

[download]

In reply to Re^2: Variable-Width Lookbehind (hacked via recursion) by haukex
in thread Variable-Width Lookbehind (hacked via recursion) by haukex

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.