GREP is a command line text search utility originally written for Unix, and the acronym means globally search a regular expression and print. Essentially, GREP in InDesign allows you to use code to find and edit or style text through the Find/Change dialog box or Paragraph Styles. GREP is complex and can be intimidating, but it’s not terribly difficult to learn the basics, and InDesign offers tools to make using GREP easy. Because of the depth of the subject, I’m only going to address GREP styles in this article.
Styles are predetermined formats for text and objects. Using styles is the fast and efficient way to manage formatting. This article gives an overview of how they work.