The match for 1 is incorrect, you can't use a character class like that. Try this test

#!perl
use strict;
use YAPE::Regex::Explain;
my $str = '&gt; milk &amp; honey &lt; < bill&ben >';

sub convert {
  my $input_stream = shift;
  return undef unless $input_stream;
  my %char_swap_hash = (
   1 => {OLD => '&[^(amp;)|(lt;)|(gt;)]', NEW => '&amp;'}, # don't mat
+ch legit '&' codes
   2 => {OLD => '<', NEW => '&lt;'},
   3 => {OLD => '>', NEW => '&gt;'}
  );
  $input_stream =~ s/$char_swap_hash{$_}{OLD}/$char_swap_hash{$_}{NEW}
+/g for keys %char_swap_hash;
  return $input_stream;
}
print YAPE::Regex::Explain->new('&[^(amp;)|(lt;)|(gt;)]')->explain;
print convert($str),"\n";

[download]

Alternative

my %htm = (
 '&' => '&amp;',
 '>' => '&gt;',
 '<' => '&lt;',
);

my $REx = qr'(&(?!(?:amp|lt|gt);)|[><])'; 
$str =~ s/$REx/$htm{$1}/g;
print $str."\n\n";
print YAPE::Regex::Explain->new($REx)->explain;

[download]
poj

In reply to Re^5: Interpolating subroutine call in SQL INSERT INTO SELECT statement by poj
in thread Interpolating subroutine call in SQL INSERT INTO SELECT statement by shadowsong

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.