<div id="tag_id" style="width: tag_width; height: tag_height;"> 
  <font face="tag_font_face" size="tag_font_size"> 
     available sizes: tag_s1, tag_s2, tag_s3; 
     available colors: tag_c1/tag_c2/tag_c3; 
     other options: tag_option1 tag_option2 tag_option3; 
  </font> 
</div>

tag_id tag_width tag_height tag_font_face tag_font_size tag_s1 tag_s2 
tag_s3 tag_c1 tag_c2 tag_c3 tag_option1 tag_option2 tag_option3

The regular expression: 
 
(?-imsx:(?:[^\w]|^)\K(tag(?:_[a-z0-9]+){1,5})(?=[^\w]|$)) 
 
matches as follows: 
   
NODE                     EXPLANATION 
---------------------------------------------------------------------- 
(?-imsx:                 group, but do not capture (case-sensitive) 
                         (with ^ and $ matching normally) (with . not 
                         matching \n) (matching whitespace and # 
                         normally): 
---------------------------------------------------------------------- 
  (?:                      group, but do not capture: 
---------------------------------------------------------------------- 
    [^\w]                    any character except: word characters 
                             (a-z, A-Z, 0-9, _) 
---------------------------------------------------------------------- 
   |                        OR 
---------------------------------------------------------------------- 
    ^                        the beginning of the string 
---------------------------------------------------------------------- 
  )                        end of grouping 
---------------------------------------------------------------------- 
  \K                       'K'  "keep" everything matched prior to \K but do not include it in $& (effectively variable-length look-behind). 
---------------------------------------------------------------------- 
  (                        group and capture to \1: 
---------------------------------------------------------------------- 
    tag                      'tag' 
---------------------------------------------------------------------- 
    (?:                      group, but do not capture (between 1 and 
                             5 times (matching the most amount 
                             possible)): 
---------------------------------------------------------------------- 
      _                        '_' 
---------------------------------------------------------------------- 
      [a-z0-9]+                any character of: 'a' to 'z', '0' to 
                               '9' (1 or more times (matching the 
                               most amount possible)) 
---------------------------------------------------------------------- 
    ){1,5}                   end of grouping 
---------------------------------------------------------------------- 
  )                        end of \1 
---------------------------------------------------------------------- 
  (?=                      look ahead to see if there is: 
---------------------------------------------------------------------- 
    [^\w]                    any character except: word characters 
                             (a-z, A-Z, 0-9, _) 
---------------------------------------------------------------------- 
   |                        OR 
---------------------------------------------------------------------- 
    $                        before an optional \n, and the end of 
                             the string 
---------------------------------------------------------------------- 
  )                        end of look-ahead 
---------------------------------------------------------------------- 
)                        end of grouping 
----------------------------------------------------------------------

#!/usr/bin/perl  
use strict;  
use warnings;  
use Data::Dumper;  
  
my $html = do { local $/ = undef; <DATA> };  
  
my @tags = $html =~ /(?:[^\w]|^)\K(tag(?:_[a-z0-9]+){1,5})(?=[^\w]|$)/ig;  
  
print Dumper \@tags;  
  
__DATA__  
<div id="tag_id" style="width: tag_width; height: tag_height;">   
  <font face="tag_font_face" size="tag_font_size">   
     available sizes: tag_s1, tag_s2, tag_s3;   
     available colors: tag_c1/tag_c2/tag_c3;   
     other options: tag_option1 tag_option2 tag_option3;   
  </font>   
</div>  
tag  
tag_  
tag_one  
tag_one_two  
tag_one_two_three  
tag_one_two_three_four  
tag_one_two_three_four_five  
tag_one_two_three_four_five_six