Perl & LWP

Chapter 10. Modifying HTML with Trees

In Chapter 9, we saw how to extract information from HTML trees. But that’s not the only thing you can use trees for. HTML::TreeBuilder trees can be altered and can even be written back out as HTML, using the as_HTML( ) method. There are four ways in which a tree can be altered: you can alter a node’s attributes; you can delete a node; you can detach a node and reattach it elsewhere; and you can add a new node. We’ll treat each of these in turn.

Changing Attributes

Suppose that in your new role as fixer of large sets of HTML documents, you are given a bunch of documents that have headings like this:

<h3 align=center>Free Monkey</h3>
<h3 color=red>Inquire Within</h3>

that need to be changed like this:

<h2 class=scream>Free Monkey</h2>
<h4 class=mutter>Inquire Within</h4>

Before you start phrasing this in terms of HTML::Element methods, you should consider whether this can be done with a search-and-replace operation in an editor. In this case, it cannot, because you’re not just changing every <h3 align=center> to <h2 class=scream> and every <h4 color=red> to <h3 class=mutter> (which are apparently simple search-and-replace operations), you also have to change </h3> to </h2> or to </h4>, depending on what you did to the element that it closes. That sort of context dependency puts this well outside the realm of simple search-and-replace operations. One could try to implement this with HTML::TokeParser, reading every token and printing it back out, after ...