GNU SED: Awesome Stream Editor

You are likely to be familiar with the "Find and Replace" dialog box from a text editor, word processor, IDE, etc to search for something and replace it with something else. sed is a command line tool that is similar, but much more versatile and feature-rich. Some of the GUI applications may also support regular expressions, a feature which helps to precisely define a matching criteria. You could consider regular expressions as a mini-programming language in itself, designed to solve various text processing needs.