Beginning Web Development With Perl : From Novice to ... - Nabo

CHAPTER 9 ■ XML PARSING WITH PERL 173On my Debian (Sarge) system, the output looks like this:--> XML::SAX::PurePerl--> XML::LibXML::SAX::Parser--> XML::LibXML::SAX--> XML::SAX::ExpatA parser is chosen through the XML::SAX::ParserFactory interface. However, in practice,programmers frequently leave it up to XML::SAX to decide which parser to use, with the defaultbeing decided by the order in which the parsers were installed. Though this may soundblatantly obvious, parsers implement functions to parse XML. The parser methodsinclude parse_uri(), parse_file(), and so on.It’s important to realize the difference, from an XML::SAX standpoint, between a parserand a handler. A parser is usually software in the form of a Perl module that is installed withXML::SAX or can be installed from CPAN. A handler, on the other hand, is software that youwrite as part of the XML parsing programming task. The parser is created or instantiated bythe XML::SAX::ParserFactory and is passed an argument telling it which handler will be used.The handler then implements interfaces for events handed to it from the parser. Note thata parser can be passed numerous arguments in addition to the name of the handler to use.XML::SAX Parser MethodsAs previously stated, a parser implements several methods for parsing XML. For most of themethods, you pass the XML as an argument, as well as other options for parsing. The parsermethods are as follows:• parse([options]): This is a generic method that can accept optional options in list,name=>value pairs, or hash format.• parse_uri(uri [, options]): This is a commonly used method to parse XML asdenoted by the URI.• parse_file(filestream [, options]): This method parses a filestream such as a filehandle.Do not confuse this method with an argument of a plain file rather thana stream.• parse_string(string [, options]): This method parses the XML contained in thestring passed to it.SAX2 Handler InterfacesThe handler that you create will need to implement code to handle events as they are passedin by the parser. XML::SAX provides access to events to ensure that the SAX2 specification ismet. XML::SAX and related parsers also work with namespaces.Logically, XML::SAX events and handlers can be grouped into categories. Many of the morecommon handlers fall into the category of content handlers. Content handlers work with theactual content of the document itself, and so content handlers are where you’ll spend a largeamount of time coding. Another important category of handlers includes the error handlers thatenable you to create custom error handling code. Other handlers include lexical handlersthat work with CDATA sections, comments, DTDs, and entities. The following sections look atcontent and error event handlers.