Learning Perl
A lot of webmasters want to learn Perl. I'm frequently asked
how to go about it.
Perl is the language of choice for most CGI programs. CGI is
a proven standard and available on essentially all internet
servers. Thousands of CGI programs are available at script
download sites, many free.
Learning a bit of Perl makes sense. It would allow you to
tweak existing scripts with some confidence. And you may
even want to write your own custom programs.
Besides, Perl is fun.
HTML Code Stripper
Consider this five-line script. It retrieves a web page from
the internet, removes all HTML tags, and displays the rest
in your browser as if it was a plain text file. Some web
pages are really funny, with long lines and short,
interspersed with blank space in odd patterns. Once the HTML
outerware is removed, all you have are the bare essentials.
#!/usr/bin/perl
use LWP::Simple;
$Page = get 'http://anydomain.com/anypage.html';
$Page =~ s/\<.+?\>//sg;
print "Content-Type: text/plain\n\n$Page";
Change the URL on line 3 to any page you want to view,
upload the script, and type the script's URL into your
browser. You'll have fun with this one.
Here are line by line comments.
Line 1 specifies the location of Perl on your server. Verify
its correctness.
Line 2 loads the Perl module LWP::Simple, which contains
methods for retrieving pages from the Internet. (If you
don't know whether or not your hosting company has
LWP::Simple installed on your server, use the
Master PreInstallation Tester to find out.)
Line 3 retrieves the URL and stores it in the variable
named $Page
Line 4 strips all HTML codes from the page. This line uses
what is known as "regular expressions" to search for and
delete everything in angle brackets.
Line 5 prints the page to your browser. It prints the header
specifying plain text, two line breaks ("\n\n"), and then
the variable $Page
If you want to retain any HTML comments on the page, change
line 4 to
$Page =~ s/\<[^!].+?\>//sg;
and the program will remove only HTML tags that do not have
an exclamation mark following the left-angle bracket.
This article is not intended to teach you Perl. The above
was intended to whet your appetite and possibly start the
creative juices flowing. Let me present just a couple more
fun things you can do (which might even be useful), then
I'll recommend a book or two and helpful links to
tutorials and such.
Snooper
If you want to make a snooper to see the source code at a
URL (supposing it's a frameset, redirecting page, external
JavaScript file, or other reason your browser can't get the
source code), replace the last two lines of the above
program with these five lines:
$Page =~ s/\</\</sg;
$Page =~ s/\>/\>/sg;
print "Content-Type: text/html\n\n";
print "<html><body><pre>$Page</pre></body></html>";
This code replaces the left- and right-angle brackets with
< and > codes. It then prints a header telling the
browser to expect an HTML formatted page. Last, it sends
the page to the browser with $Page between PRE tags.
Personalized Web Pages
The last example, a bit more complicated, is a bare-bones
method of delivering personalized web pages.
First, place some tags into a web page. (If you put the tags
between angle brackets, the page can be displayed without
personalizing, too.) Here is an example page to get you
started.
<html>
<body>
<p>Hello <MyTag:name> — this is your personal
page. If we don't have your correct email address
<MyTag:email> then please send it to us. Thank
You <MyTag:name> !!!</p>
</body>
</html>
With the tags placed as they are, the page can be displayed
without personalization -- the tags just don't show up.
To display that page with personalization, link to this
script instead of directly to the above page:
#!/usr/bin/perl
use LWP::Simple;
$Name = 'William';
$Addy = 'possibilities@willmaster.com';
$Page = get 'http://url/to/above/page.html';
$Page =~ s/\<MyTag\:name\>/$Name/sig;
$Page =~ s/\<MyTag\:email\>/$Addy/sig;
print "Content-Type: text/html\n\n$Page";
In an efficient system, the name and email address would be
retrieved from a database or cookie. In this example, lines
3 and 4 store the name and email address into the variables
named $Name and $Addy
Line 5 must contain the URL to your template page you
created above.
Lines 6 and 7 replace the tags with your custom information.
And line 8 sends the web page to the browser.
A Way of Thinking
Programming requires a certain way of thinking, just as
gardening and writing a novel each require their own ways
of thinking. The description may be inadequate, but writing
a program line requires focusing on a tiny detail while
imagining consequences several steps removed and visualizing
the project as a whole. It helps to remember you're writing
something for a machine that "thinks" only with zeros and
ones, that every thing is either entirely true or entirely
false.
The way to learn is to practice programming. The more you
program, the more you become familiar with the thought
process.
Getting Started
Obtain a "learning perl" book, to start, and never mind all
the other resources until later. There's a lot of stuff out
there, and it could be a bit confusing without a basic
grounding in the language.
I recommend "Perl for Dummies (with CD ROM)" by Paul Hoffman.
The CD contains a Perl interpreter to install on your
computer. Mari learned Perl with this book. Her previous
programming experience was a class with BASIC or FORTRAN
in high school. (A similar title by the same author,
"Perl 5 for Dummies," is also on bookshelves. I've never
read that one, so can't make a recommendation.)
Do each exercise, from front to back, and you'll be well on
your way to Perl guru-ness.
Some people learn well with "Learning Perl" by Randal L.
Schwartz & Tom Christiansen. If you have a programming
background, you may want to use this one instead.
If you need to pick up a copy of Perl for your computer,
http://cpan.org/ports/ contains links where you can find
Perl for dozens of operating systems, including Windows
and Macintosh. Windows users have several choices; I
recommend ActiveState (use the MSI link) at
http://aspn.activestate.com/ASPN/Downloads/ActivePerl/
See "CGI Developer's Tools" linked from the
WillMaster Possibilities article archives index at
/library/
for tools you may wish to obtain for yourself.
Other Resources
After you become comfortable with the language, you'll want
to become familiar with other Perl resources.
You'll find tutorials and helpful documentation at these
three links:
- http://cgi.resourceindex.com/Documentation/
- http://www.hotscripts.com/Perl/Tips_and_Tutorials/
- http://www.perl.com/pub/q/documentation
A page of bookmarks to Perl related resources is at
http://bookmarks.cpan.org/
http://learn.perl.org/ has Perl books listed for various
programming skill levels, and they have some lists you
might consider joining.
http://perl.com/ is a technical book publisher's site with
focus on Perl. It has numerous resource links.
Mailing lists, more than you'll ever need can be found at
http://lists.perl.org/
A technical FAQ can be found at
http://www.cpan.org/misc/cpan-faq.html
http://www.perl.com/pub/q/Article_Archive contains links to
many articles, including programming how-to.
What do I use? My favorite reference book is "Programming
Perl" by Larry Wall, Tom Christiansen & Randal L. Schwartz.
It's dog-eared and the covers are curling. I use it every
day.
Will Bontrager
©2001 Bontrager Connection, LLC
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