CS 5500 Assignment #3.
Assigned: Thursday, 16 January 2014
Due: Thursday, 30 January 2014

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 or .tgz.  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/cs5500sp14/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 (apart from standard input)
        not attempting to create any files (apart from standard output)
        not producing extraneous output
        having no side effects apart from reading from and writing to
            standard input and output
        producing the specified error message for incorrect inputs
        returning an exit status of zero for correct inputs
        returning a nonzero exit status for incorrect inputs
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

Your check5500 program must then read its input from standard input
and decide whether that input is a syntactically correct message,
where the syntax of correct messages is specified by the following
context-free grammar:

    <message>     ::=  <optWS> <msg> <optWS>

    <msg>         ::=  <integer>
                    |  <identifier>
                    |  <string>
                    |  <lparen> <optWS> <rparen>
                    |  <lparen> <optWS> <messages> <optWS> <rparen>

    <messages>    ::=  <msg>
                    |  <msg> <whitespace> <messages>

    <integer>     ::=  <digit>
                    |  <digit> <integer>
    <identifier>  ::=  <letter>
                    |  <identifier> <letter>
                    |  <identifier> <digit>
    <string>      ::=  <doublequote> <characters> <doublequote>

    <optWS>       ::=
                    |  <whitespace> <optWS>
    <whitespace>  ::=  <whitechar>
                    |  <whitechar> <whitespace>

    <digit>       ::=  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
    <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
    <doublequote> ::=  "
    <characters>  ::=
                  ::=  <character> <characters>
    <character>   ::=  <digit>
                    |  <letter>
                    |  <space>
    <whitechar>   ::=  <space>
                    |  <newline>

If your software discovers that its input is syntactically 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 input is syntactically 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 inputs should produce no output and terminate with
an exit status of 0:

    0010

    Helen

    "In Xanadu did Kubla Khan"

    ()

            (   )

    (start)

    ( end win )

    (move (A3 B3))

    (attack (D1 E1)
            (result attacker))

    (timing 95 60 120)

    (() (()) ((())) (((()))) ((((())))))

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

    10x

    D'Artagnan

    "No, punctuation is not allowed within strings."

    () )

    (

    (start"finish")

    (end) (win)

    (move "(A3 B3)")

    (attack )D1 E1( (result attacker))

    (()(())((()))(((())))((((())))))

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