prints lines from each
that match the
a regular expression as defined in
If no files are given, standard input is used.
If more than one file is given, each line of output
is preceded by the name of the file it was found in.
The options are:
Print only the name of each file that contains a match.
Print the name of each file that does not contain a match.
Precede each line of output with its line number.
Print only lines that do not match
Pattern matching is case insensitive (roman letters only).
Do not print anything; yield only the exit status.