UPDATE: Read Regexes are slow (or, why I advocate String::Index) for a detailed explanation of the general problem with regexes here.
There's no need to use a regex: 
if (length($str) > 1000) {
  substr($str, 1+rindex($str, '.', 1000)) = "";
}

[download]
See the rindex function's docs. It returns the last location of the substring (here, ".") in the string ($str). We're telling it to start looking at the 1000th character (and work backwards).

If you want to allow various punctuation, might I suggest my String::Index module? It's faster than the typical regex solution and a hybrid regex/substr solution. 

#!/usr/bin/perl

use Benchmark 'cmpthese';
use String::Index 'crindex';

my $str = "alphabet. alphabet! alphabet? " x 100;

cmpthese(-5, {
  rcindex => sub {
    my $x = $str;
    substr($x, 1+crindex($str, ".!?", 1000)) = "";
  },
  regex => sub {
    my $x = $str;
    $x =~ s/^(.{1,999}[.!?]).*/$1/;
  },
  rxsubstr => sub {
    my $x = $str;
    $x =~ /^.{1,999}[.!?]/ and substr($x, $+[0]) = "";
  },
});

__END__
             Rate    regex rxsubstr  rcindex
regex     42520/s       --     -43%     -66%
rxsubstr  75202/s      77%       --     -40%
rcindex  125559/s     195%      67%       --

[download]
String::Index gives you four functions that are crosses between Perl's index() function and C's strpbrk() function.

(I need to fix the docs or the module a tad. The function is 'crindex', but I have 'rcindex' somewhere.)

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

In reply to Re: Truncating Last Sentence by japhy
in thread Truncating Last Sentence by FatDog

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.