Chapter 10, and this search could be exposed as a enhanced like method, such as the patternLike used in Chapter 13.
The Document mechanism is a flexible one and need not correspond exactly to a file, URL, or database row. A document could span several files by including a field that lists the component documents. Alternately a document could consist of a single mail message in a file containing an archive of many such messages by including fields containing the start and end lines.
General free text search is a surprisingly powerful capability, and some thought may suggest ways in which it can enhance any number of applications.