PowerGREP is a versatile and powerful text processing and search tool based on regular expressions. A regular expression is a pattern that describes the form of a piece of text. A regular expression could match a date or an email address, for example. Any date or any email address that is, without specifying actual dates or actual email addresses. Your search patterns can be as specific or as general as you want. This makes PowerGREP much more flexible than a general search tool that only finds words and phrases (PowerGREP can do that too).
With PowerGREP you can use one or more such regular expressions to get lists of files, lists of search matches in files, search-and-replace through files, rename files, merge files, split files, etc. First read the "how to use PowerGREP" section to get a feel of the way PowerGREP works. Then check out the examples that seem interesting to you. All examples include step-by-step instructions. The examples don't require prior experience with PowerGREP, but you'll understand them better if you check out the "how to" section first.