When you write a backtracking regex engine like the one in Perl 5, a full (variable length) look-ahead is pretty easy to implement.

It seems to me that the easy solution would produce bad performance. The easy solution requires matching each lookbehind subpattern at pos $current_pos - $max_match_length, which is 0 if /.*/ is used! 

'bbbbbb' =~ /(?<=[^z]*a)b/

[download]

requires checking for "z" and "a" a total of 72 times each before failing even though the string is only 6 chars long!

Oops, I guess you covered this at the end, but it seems to me that it's a foremost concern.

It also forces 

'xiyjykz' =~ /(?<=x(.*)y(.*))z/s

[download]
to produce 
$1 = 'iyj';  $2 = 'k'

[download]
but one might expect 
$1 = 'i';  $2 = 'jyk'

[download]

In reply to Re^2: Why is variable length lookahead implemented while lookbehind is not? by ikegami
in thread Why is variable length lookahead implemented while lookbehind is not? by Kc12349

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.