# Grep

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

The Unix [`grep`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html) 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:

```text
hello
world
hello again
```

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

```text
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:

```text
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:

```text
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".

## Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
`raise Exception`, you should write:

```python
raise Exception("Meaningful message indicating the source of the error")
```

## Running the tests

To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):

- Python 2.7: `py.test grep_test.py`
- Python 3.3+: `pytest grep_test.py`

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
`python -m pytest grep_test.py`

### Common `pytest` options

- `-v` : enable verbose output
- `-x` : stop running tests on first failure
- `--ff` : run failures from previous test before running other test cases

For other options, see `python -m pytest -h`

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/grep` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.

For more detailed information about running tests, code style and linting,
please see the [help page](http://exercism.io/languages/python).

## Source

Conversation with Nate Foster. [http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf](http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf)

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.