#!/usr/bin/perl -wl

### this code assumes that there are no characters
### in the "ignore_last" class that AREN'T in the
### "word" class -- it might seem silly that there
### would be, but still, that's how I'm coding this  
  
use strict;
  
my $text_stream = q{foo#&#bar};

my $ig_first = '#'; 
my $ig_last = '';
my $word = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#';
    
    
my $pre    = length($ig_first) ? qr/[\Q$ig_first\E]*/ : '';
my $post   = length($ig_last)  ? qr/[\Q$ig_last\E]*/  : '';
my $inside = length($ig_last)  ? qr/[\Q$ig_last\E]+/  : '';
my ($match, @words);
      
         
{
  # remove chars from $word that are matched by $post
  my $reg = $word;
  $reg =~ s/$post+//g if $post;
  $reg = qr/[\Q$reg\E]/;
 

  # unroll the loop:
  $match = qr{
    ($pre)        # pre chars (save to $1)

    (             # (save to $2)
      $reg+       # one or more regular chars

      |           # OR

      $reg*       # zero or more regular chars
      (?:
        $inside   # one or more post chars
        $reg+     # one or more non-post chars
      )+          # this chunk one or more times
    )
  }x;             # /x for extended mode
}

    
$text_stream =~ s[$match]{ push @words, $2; "$1<b>$2</b>" }eg;

print $text_stream;
print "words: @words";