In many cases, it's useful to allow users to define patterns or large chunks of text that contain programmatically derived values. For example, from email messages addressed to a given user, or URL path aliases containing the title of a given node. Both examples require bits of data that vary each time the text is generated -- node titles, user ids, and so on. Rather than forcing users to embed ugly snippets of PHP, or creating elaborate and bizarre UIs for configuring the patterns via the browser, it's most useful to give users a set of 'placeholder' tokens to place in their text.