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Using templates with Perl/CGI

  • December 14, 1998
  • By Dave Edis
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Most Web applications by nature use a lot of HTML. Think about a Web-based email program such as Hotmail. HTML is used to display numerous pages including the login screen, mailbox, read message, send message, folders, menubar, setup options, and many other pages.

The problem with having HTML in Web programs is that there's just so much of it. Every single screen in your program needs an HTML page. And where does all that HTML usually go? In your Perl source code. This can add literally thousands of lines of code making it difficult to scan and troubleshoot your programs, difficult to see each individual screen without cutting and pasting it into its own file, and even more difficult to edit those HTML screens, let alone have some one else edit them. The solution? Put that HTML where it belongs, in ".HTML" files! And write your Perl/CGI programs to make use make use of these files as HTML Templates.

Finding a new home for all that HTML

The best way to organize your HTML Templates is to put them near your program (either in the same directory or a sub directory named "/templates/") and to give them a good descriptive name. I usually make the first character of the filename an underscore followed by the program name and the program section. I use the underscore at the beginning to differentiate between regular HTML files and HTML templates.

For example, pretend we were to create a Web-based email program called "Web Mail", our templates may be named as follows: 

_webmail_login.html                  # login 
_webmail_message_list.html           # list messages 
_webmail_message_read.html           # read message 
_webmail_message_send.html           # create/send message 
_webmail_message_erase.html          # erase message
_webmail_message_erase_confirm.html  # confirm erase 
_webmail_logoff.html                 # logoff

This makes it easy to see at a glance what the file is for and even easier to come back later and make any changes or see what a specific page looks like. Also, if you work with other web developers and designers, they can make changes without fear or messing up your program by accident or remembering to backslash all the @ symbols.

Displaying templates in Perl/CGI

Before we can start moving all that HTML out of our Perl/CGI programs and into HTML template files we need some way to display them. So, we're going to create a Perl subroutine called 
&Template
. Here's the code: 

sub Template { 
local(*FILE);         # filehandle 
local($file);         # file path 
local($HTML);         # HTML data 

$file = $_[0] || die "Template : No template file specified\n"; 

open(FILE, "<$file") || die "Template : Couldn't open $file : $!\n";
while () { $HTML .= $_; }
close(FILE);

$HTML =~ s/\$(\w+)/${$1}/g; 

return $HTML; 
}

Most of this is pretty basic, we're just opening a template file, reading the contents and passing it back to the program. The trick is the following regular expression that searches for Perl variables in your HTML template (i.e., "$date") and substitutes them with the value of the that variable as defined in your program:

$HTML =~ s/\$(\w+)/${$1}/g;





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