CS 5500 Assignment #3.
Assigned: Wednesday, 11 September 2013
Due: Wednesday, 25 September 2013

This is an individual assignment.  Collaboration between students
is forbidden on this assignment.  You are responsible for keeping
your code hidden from all other students.

For this assignment, you will write a simple syntax checker
that implements the specification given below.  You must
write all of your program's source code by yourself.  You
are not allowed to use source code written by other students,
you are not allowed to use source code obtained from the
World-Wide Web or other sources, and you are not allowed
to use software packages unless they are already installed
on the standard CCIS Linux machines.

Your software must include a README file (which *must* be
in UTF-8 plain text, and must be named README) that

1.  gives your name (as you want the instructor to spell it),
2.  gives your preferred email address(es) for contacting you,
3.  tells the grader(s) how your software can be compiled and
    run on any CCIS Linux machine in the main lab.

All of the files necessary to construct and to run your
software must be combined into a gzip'ed tar file whose
name ends in tar.gz.  Submit that gzip'ed tar file before
6pm on the date it is due using the submit script that's
described at the course assignments page:

    http://www.ccs.neu.edu/course/cs5500f13/assignments.html

Your software will be graded on these criteria:

1.  the quality of the instructions and documentation
    in your README file,
2.  the ease of constructing and running your software on
    CCIS Linux machines,
3.  your software's correctness with respect to
        not attempting to read any files
        not attempting to create any files
        not producing extraneous output
        having no side effects apart from writing to standard output
        producing the specified error message for incorrect inputs
        returning an exit status of zero for correct command lines
        returning a nonzero exit status for incorrect command lines
4.  and the readability of your source code.

Your software's build process must result in software that
can be invoked by cd'ing to the directory containing your
software's executable and executing a command of the following
form

    ./check5500 <pathspec> <pathspec>

where <pathspec> is a sequence of tokens generated by

    <pathspec>  ::=  -f <pathname>
                  |  --file <pathname>
                  |  -d <pathname>
                  |  --dir <pathname>

and <pathname> is a sequence of ASCII characters generated by

    <pathname>    ::=  <pathchar>
                    |  <pathchar> <pathname>
    <pathchar>    ::=  <slash>
                    |  <period>
                    |  <tilde>
                    |  <hyphen>
                    |  <underscore>
                    |  <letter>
                    |  <digit>
    <slash>       ::=  /
    <period>      ::=  .
    <tilde>       ::=  ~
    <hyphen>      ::=  -
    <underscore>  ::=  _
    <letter>      ::=  a | b | c | d | e | f | g | h | i | j | k | l | m |
                       n | o | p | q | r | s | t | u | v | w | x | y | z |
                       A | B | C | D | E | F | G | H | I | J | K | L | M |
                       N | O | P | Q | R | S | T | U | V | W | X | Y | Z
    <digit>       ::=  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Note that <pathname> as specified here is not quite the same
as a Linux path name, so you will have to write your own code
to check the syntax of a <pathname>.

Most programming languages provide a built-in way to split the
command line into tokens, and you may rely on that built-in
tokenizer when parsing the command line and each <pathspec>.

If your software discovers that its command line was correct
according to the specification above, then your software should
produce no output at all and should terminate with an exit status
of 0.

If your software discovers that its command line was incorrect
according to the specification above, then your software should
write "ERROR" to standard output, followed by a newline (also
sent to standard output), and should terminate with an exit
status other than 0.

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Examples.

The following test cases should produce no output and terminate
with an exit status of 0:

    ./check5500 -f here -d there

    ./check5500 --dir /~~/./..////.../_~-..// -d -

    ./check5500 --file a -f b

The following test cases should write "ERROR" to standard output
and should terminate with an exit status other than 0:

    ./check5500

    ./check5500 -f

    ./check5500 -f here

    ./check5500 -f here -d

    ./check5500 -f here -d there --dir everywhere

    ./check5500 -file here -d there

    ./check5500 -f here -dir there

    ./check5500 -f x$50 -d yz

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