lima1 wants minimal matches, so the {49,} should actually be {49,}?, in which case it stops working.

The reason is that the look-ahead is not limited to what the [\w\s]{49,}? matches. A small demonstration: 

#!/usr/bin/perl
use strict;
use warnings;

my $re = qr{^\s*(?=\w+\s+\w+)[\w\s]{49,}?\w};

my $str =  ('x' x 65) . ' x';

if ($str =~ m/$re/) {
    print $&, $/;
}
__END__
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

[download]

You see that match only includes a long word, not a sentence. There is a sentence present, but it's not matched.

Update: In Perl 6 you could use & like this: 

regex short_word {
   \s* 
   [ 
   & \w+ \s+ \w+ 
   & .**?{1..50}
   ] 
}

[download]

In reply to Re^2: RE question: Sentence with a minimum length by moritz
in thread RE question: Sentence with a minimum length by lima1

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.