Grep

Search a file for lines matching a regular expression pattern. Return the line
number and contents of each matching line.

The Unix grep command can be used to search for lines in one or more files
that match a user-provided search query (known as the pattern).

The grep command takes three arguments:

1. The pattern used to match lines in a file.
2. Zero or more flags to customize the matching behavior.
3. One or more files in which to search for matching lines.

Your task is to implement the grep function, which should read the contents
of the specified files, find the lines that match the specified pattern
and then output those lines as a single string. Note that the lines should
be output in the order in which they were found, with the first matching line
in the first file being output first.

As an example, suppose there is a file named "input.txt" with the following contents:

hello
world
hello again

If we were to call grep "hello" input.txt, the returned string should be:

hello
hello again

Flags

As said earlier, the grep command should also support the following flags:

• -n Print the line numbers of each matching line.
• -l Print only the names of files that contain at least one matching line.
• -i Match line using a case-insensitive comparison.
• -v Invert the program -- collect all lines that fail to match the pattern.
• -x Only match entire lines, instead of lines that contain a match.

If we run grep -n "hello" input.txt, the -n flag will require the matching
lines to be prefixed with its line number:

1:hello
3:hello again

And if we run grep -i "HELLO" input.txt, we'll do a case-insensitive match,
and the output will be:

hello
hello again

The grep command should support multiple flags at once.

For example, running grep -l -v "hello" file1.txt file2.txt should
print the names of files that do not contain the string "hello".