What is grep used for?

Context: grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p, which has the same effect. "grep" was originally developed for the Unix operating system, but later available for all Unix-like systems and some others such as OS-9.

Answer:  Grep is used for searching plain-text data sets for lines that match a regular expression