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

CHAPTER 9■ ■ ■XML Parsing with PerlYou have some data in XML. Maybe that data is from a SOAP web service, maybe it’s from anRSS feed, or maybe it’s from another source. Now you want to read the XML and extract the datafrom it. As is the theme with Perl, you have multiple ways to accomplish this task.XML parsing with Perl has a storied history. Early modules were quirky, while others wereincomplete.Parsing simple XML with Perl is, well, simple. Parsing complex XML with Perl can be quitedifficult. The important thing to remember is that XML is just a way to represent data. That datahappens to be in an XML document. The program that you write to parse XML will first needto read the XML, and then use the results as it would any other data input.This chapter looks at XML parsing with Perl. It first reviews the main parsing methods,and then describes using two modules: XML::Simple and XML:SAX. Finally, it examines treebasedparsing.XML Parsing MethodsRecall that there’s always more than one way to do the same thing with Perl. XML parsing isno different. And, of course, there’s no rule that says that you must use an XML parser at all.It’s quite possible for you to write your own XML parser, just as it would be possible to writeyour own module for anything in Perl, rather than using an already existing module.Primarily, two methods exist for parsing an XML document:Stream parsing: Stream-based parsers process XML as it is read into the parser. As new elementsare encountered (which are called tokens), they are processed by the parser and sentinto your program through a process of events. This means that the program must processeach piece of data as it is encountered by the stream-based parser. Stream-based parsershave lower memory requirements than their tree-based counterparts, simply because theydon’t store any data; rather, they send data along into the rest of the program as it is found.Of course, the lower memory requirements are gained at the expense of complexity whencompared to tree-based parsers.Tree parsing: Tree-based parsers load entire XML structures into memory for later processing.This means that the entire document is parsed prior to your program needing to handle it. Inturn, this leads to less complex programs when compared to stream-based parsing. The extrasimplicity comes at the cost of higher memory requirements. Naturally, on a modern computerwith a small document to parse, the memory required will be minimal.165