REX is most useful is when you're focusing on lexical details of a document -- for example, when transforming one kind of text document (e.g., plain text, XML,

REX is most useful is when you're focusing on lexical details of a document -- for example, when transforming one kind of text document (e.g., plain text, XML, SGML, HTML) into another, where the document may not be valid, well formed, or even parsable for most of the transformation. It lets you target islands of markup anywhere within a document without disturbing the rest of the document.