20.0. Introduction

Chapter 19, CGI Programming , concentrated on responding to browser requests and producing documents using CGI. This one approaches the Web from the other side: instead of responding to a browser, you pretend to be one, generating requests and processing returned documents. We make extensive use of modules to simplify this process, because the intricate network protocols and document formats are tricky to get right. By letting existing modules handle the hard parts, you can concentrate on the interesting part - your own program.

The relevant modules can all be found under the following URL:

http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/

There are modules for computing credit card checksums, interacting with Netscape or Apache server APIs, processing image maps, validating HTML, and manipulating MIME. The largest and most important modules for this chapter, though, are found in the libwww-perl suite of modules, referred to collectively as LWP. Here are just a few of the modules included in LWP:

Module Name	Purpose
LWP::UserAgent	WWW user agent class
LWP::RobotUA	Develop robot applications
LWP::Protocol	Interface to various protocol schemes
LWP::Authen::Basic	Handle 401 and 407 responses
LWP::MediaTypes	MIME types configuration (text/html, etc.)
LWP::Debug	Debug logging module
LWP::Simple	Simple procedural interface for common functions
HTTP::Headers	MIME/RFC822 style headers
HTTP::Message	HTTP style message
HTTP::Request	HTTP request
HTTP::Response	HTTP response
HTTP::Daemon	A HTTP server class
HTTP::Status	HTTP status code (200 OK etc)
HTTP::Date	Date parsing module for HTTP date formats
HTTP::Negotiate	HTTP content negotiation calculation
WWW::RobotRules	Parse robots.txt files
File::Listing	Parse directory listings

The HTTP:: and LWP:: modules let you request documents from a server. The LWP::Simple module, in particular, offers a very basic way to fetch a document. LWP::Simple, however, lacks the ability to access individual components of the HTTP response. To access these, use HTTP::Request, HTTP::Response, and LWP::UserAgent. We show both sets of modules in Recipes Recipe 20.1 , Recipe 20.2 , and Recipe 20.10 .

Closely allied with LWP, but not distributed in the LWP bundle, are the HTML:: modules. These let you parse HTML. They provide the basis for Recipes Recipe 20.5 , Recipe 20.4 , Recipe 20.6 , Recipe 20.3 , Recipe 20.7 , and the programs htmlsub and hrefsub.

Recipe 20.12 gives a regular expression to decode the fields in your web server's log files and shows how to interpret the fields. We use this regular expression and the Logfile::Apache module in Recipe 20.13 to show two ways of summarizing the data in web server log files.