A user interface, a programmable web browser and text editor that demonstrates how lightweight structure can be useful.

From its home page (see below):

Lightweight Structure is the ability to recognize text structure automatically, using an extensible library of patterns and parsers. Structure can be detected in lots of ways: grammars (e.g. Java or HTML), regular expressions, even manual selections by the user. With lightweight structure, it doesn't matter how the structure was detected--whether by a regular expression, or by a grammar, or by a hand-coded parser. All that matters is the result: a region set, which is a set of intervals in the text.

It features (excerpt from the home page):

This page is linked from: Metatext