Many commands support the regular expression syntax, but the most commonly used is the command grep, which is designed to display lines from a file or files matching a given regular expression.
There are multiple ways to call grep to change its behavior:
Treat the pattern as a basic regular expression.
Treat the pattern as an extended regular expression. Same as grep –E.
Treat the pattern as a list of fixed strings, any of which may be matched. Same as grep –F.