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\section*{Coursework 1}

This coursework is worth 5\% and is due on \cwONE{} at 16:00. You are

asked to implement a regular expression matcher and submit a document

containing the answers for the questions below. You can do the

implementation in any programming language you like, but you need to

submit the source code with which you answered the questions,

otherwise a mark of 0\% will be awarded. You can submit your answers

in a txt-file or pdf.  Code send as code. Please package everything

inside a zip-file that creates a directory with the name

\[\texttt{YournameYourfamilyname}\]

\noindent on my end. Thanks!

\subsubsection*{Disclaimer\alert}

It should be understood that the work you submit represents

your \textbf{own} effort. You have not copied from anyone else. An

exception is the Scala code I showed during the lectures or

uploaded to KEATS, which you can freely use. Do not

be tempted to ask Github Copilot for help or do any other

shenanigans like this!\bigskip

\noindent

If you have any questions, please send me an email in \textbf{good}

time.\bigskip

\subsection*{Task}

The task is to implement a regular expression matcher based on

derivatives of regular expressions. The implementation should

be able to deal with the usual (basic) regular expressions

\[

\ZERO,\; \ONE,\; c,\; r_1 + r_2,\; r_1 \cdot r_2,\; r^*

\]

\noindent

but also with the following extended regular expressions:

\begin{center}

\begin{tabular}{ll}

  $[c_1,c_2,\ldots,c_n]$ & a set of characters---for character ranges\\

  $r^+$ & one or more times $r$\\

  $r^?$ & optional $r$\\

  $r^{\{n\}}$ & exactly $n$-times\\

  $r^{\{..m\}}$ & zero or more times $r$ but no more than $m$-times\\

  $r^{\{n..\}}$ & at least $n$-times $r$\\

  $r^{\{n..m\}}$ & at least $n$-times $r$ but no more than $m$-times\\

  $\sim{}r$ & not-regular-expression of $r$\\

\end{tabular}

\end{center}

\noindent You can assume that $n$ and $m$ are greater or equal than

$0$. In the case of $r^{\{n,m\}}$ you can also assume $0 \le n \le m$.\bigskip

\noindent {\bf Important!} Your implementation should have explicit

case classes  for the basic regular expressions, but also explicit case

classes for

the extended regular expressions.\footnote{Please call them

  \code{RANGE}, \code{PLUS}, \code{OPTIONAL}, \code{NTIMES},

  \code{UPTO}, \code{FROM} and \code{BETWEEN}.} 

  That means do not treat the extended regular expressions

by just translating them into the basic ones. See also Question 3,

where you are asked to explicitly give the rules for \textit{nullable}

and \textit{der} for the extended regular expressions. Something like

\[der\,c\,(r^+) \dn der\,c\,(r\cdot r^*)\]

\noindent is \emph{not} allowed as answer in Question 3 and \emph{not}

allowed in your code.\medskip

\noindent

The meanings of the extended regular expressions are

\begin{center}

\begin{tabular}{r@{\hspace{2mm}}c@{\hspace{2mm}}l}

  $L([c_1,c_2,\ldots,c_n])$ & $\dn$ & $\{[c_1], [c_2], \ldots, [c_n]\}$\\ 

  $L(r^+)$                  & $\dn$ & $\bigcup_{1\le i}.\;L(r)^i$\\

  $L(r^?)$                  & $\dn$ & $L(r) \cup \{[]\}$\\

  $L(r^{\{n\}})$             & $\dn$ & $L(r)^n$\\

  $L(r^{\{..m\}})$           & $\dn$ & $\bigcup_{0\le i \le m}.\;L(r)^i$\\

  $L(r^{\{n..\}})$           & $\dn$ & $\bigcup_{n\le i}.\;L(r)^i$\\

  $L(r^{\{n..m\}})$          & $\dn$ & $\bigcup_{n\le i \le m}.\;L(r)^i$\\

  $L(\sim{}r)$              & $\dn$ & $\Sigma^* - L(r)$

\end{tabular}

\end{center}

\noindent whereby in the last clause the set $\Sigma^*$ stands

for the set of \emph{all} strings over the alphabet $\Sigma$

(in the implementation the alphabet can be just what is

represented by, say, the type \pcode{Char}). So $\sim{}r$

means in effect ``all the strings that $r$ cannot match''.\medskip 

\noindent

Be careful that your implementation of \textit{nullable} and

\textit{der} satisfies for every regular expression $r$ the following

two properties (see also Question 3):

\begin{itemize}

\item $\textit{nullable}(r)$ if and only if $[]\in L(r)$

\item $L(der\,c\,r) = Der\,c\,(L(r))$

\end{itemize}

\subsection*{Question 1 (Unmarked)}

What is your King's email address (you will need it in

Question 5)? Also could you please let me know from where you will be mainly

studying? (online / in-person, in London / somewhere else) Thanks!

\subsection*{Question 2 (Unmarked)}

Can you please also list all programming languages in which you have

already written programs (include only instances where you have spent

at least a good working day fiddling with a program)?  This is just

for my curiosity to estimate what your background is.

\subsection*{Question 3}

From the

lectures you have seen the definitions for the functions

\textit{nullable} and \textit{der} for the basic regular

expressions. Implement and write down the rules for the extended

regular expressions:

\begin{center}

\begin{tabular}{@ {}l@ {\hspace{2mm}}c@ {\hspace{2mm}}l@ {}}

  $\textit{nullable}([c_1,c_2,\ldots,c_n])$  & $\dn$ & $?$\\

  $\textit{nullable}(r^+)$                   & $\dn$ & $?$\\

  $\textit{nullable}(r^?)$                   & $\dn$ & $?$\\

  $\textit{nullable}(r^{\{n\}})$              & $\dn$ & $?$\\

  $\textit{nullable}(r^{\{..m\}})$            & $\dn$ & $?$\\

  $\textit{nullable}(r^{\{n..\}})$            & $\dn$ & $?$\\

  $\textit{nullable}(r^{\{n..m\}})$           & $\dn$ & $?$\\

  $\textit{nullable}(\sim{}r)$              & $\dn$ & $?$

\end{tabular}

\end{center}

\begin{center}

\begin{tabular}{@ {}l@ {\hspace{2mm}}c@ {\hspace{2mm}}l@ {}}

  $der\, c\, ([c_1,c_2,\ldots,c_n])$  & $\dn$ & $?$\\

  $der\, c\, (r^+)$                   & $\dn$ & $?$\\

  $der\, c\, (r^?)$                   & $\dn$ & $?$\\

  $der\, c\, (r^{\{n\}})$              & $\dn$ & $?$\\

  $der\, c\, (r^{\{..m\}})$           & $\dn$ & $?$\\

  $der\, c\, (r^{\{n..\}})$           & $\dn$ & $?$\\

  $der\, c\, (r^{\{n..m\}})$           & $\dn$ & $?$\\

  $der\, c\, (\sim{}r)$               & $\dn$ & $?$\\

\end{tabular}

\end{center}

\noindent

Remember your definitions have to satisfy the two properties

\begin{itemize}

\item $\textit{nullable}(r)$ if and only if $[]\in L(r)$

\item $L(der\,c\,r)) = Der\,c\,(L(r))$

\end{itemize}

\noindent

Given the definitions of \textit{nullable} and \textit{der}, it is

easy to implement a regular expression matcher.  Test your regular

expression matcher with (at least) the examples:

\begin{center}

\def\arraystretch{1.2}  

\begin{tabular}{@{}r|m{3mm}|m{6mm}|m{6mm}|m{10mm}|m{6mm}|m{10mm}|m{10mm}|m{10mm}}

  string & $a^?$ & $\sim{}a$ & $a^{\{3\}}$ & $(a^?)^{\{3\}}$ & $a^{\{..3\}}$ &

     $(a^?)^{\{..3\}}$ & $a^{\{3..5\}}$ & $(a^?)^{\{3..5\}}$ \\\hline

  $[]$           &&&&&&& \\\hline 

  \texttt{a}     &&&&&&& \\\hline 

  \texttt{aa}    &&&&&&& \\\hline 

  \texttt{aaa}   &&&&&&& \\\hline 

  \texttt{aaaaa} &&&&&&& \\\hline 

  \texttt{aaaaaa}&&&&&&& \\

\end{tabular}

\end{center}

\noindent

Does your matcher produce the expected results? Make sure you

also test corner-cases, like $a^{\{0\}}$!

\subsection*{Question 4}

As you can see, there are a number of explicit regular expressions

that deal with single or several characters, for example:

\begin{center}

\begin{tabular}{ll}

  $c$ & matches a single character\\  

  $[c_1,c_2,\ldots,c_n]$ & matches a set of characters---for character ranges\\

  $\textit{ALL}$ & matches any character

\end{tabular}

\end{center}

\noindent

The latter is useful for matching any string (for example

by using $\textit{ALL}^*$). In order to avoid having an explicit constructor

for each case, we can generalise all these cases and introduce a single

constructor $\textit{CFUN}(f)$ where $f$ is a function from characters

to booleans. In Scala code this would look as follows:

\begin{lstlisting}[numbers=none]

abstract class Rexp

...

case class CFUN(f: Char => Boolean) extends Rexp 

\end{lstlisting}\smallskip

\noindent

The idea is that the function $f$ determines which character(s)

are matched, namely those where $f$ returns \texttt{true}.

In this question implement \textit{CFUN} and define

\begin{center}

\begin{tabular}{@ {}l@ {\hspace{2mm}}c@ {\hspace{2mm}}l@ {}}

  $\textit{nullable}(\textit{CFUN}(f))$  & $\dn$ & $?$\\

  $\textit{der}\,c\,(\textit{CFUN}(f))$  & $\dn$ & $?$

\end{tabular}

\end{center}

\noindent in your matcher and then also give definitions for

\begin{center}

\begin{tabular}{@ {}l@ {\hspace{2mm}}c@ {\hspace{2mm}}l@ {}}

  $c$  & $\dn$ & $\textit{CFUN}(?)$\\

  $[c_1,c_2,\ldots,c_n]$  & $\dn$ & $\textit{CFUN}(?)$\\

  $\textit{ALL}$  & $\dn$ & $\textit{CFUN}(?)$

\end{tabular}

\end{center}

\noindent

You can either add the constructor $CFUN$ to your implementation in

Question 3, or you can implement this questions first

and then use $CFUN$ instead of \code{RANGE} and \code{CHAR} in Question 3.

In an ideal world one would do this task first, but this might confuse

you with what you need to do in the previous question.

\subsection*{Question 5}

Suppose $[a\mbox{-}z0\mbox{-}9\_\,.\mbox{-}]$ stands for the regular expression

\[[a,b,c,\ldots,z,0,\dots,9,\_,.,\mbox{-}]\;.\]

\noindent

Define in your code the following regular expression for email addresses

\[

([a\mbox{-}z0\mbox{-}9\_\,.-]^+)\cdot @\cdot ([a\mbox{-}z0\mbox{-}9\,.-]^+)\cdot .\cdot ([a\mbox{-}z\,.]^{\{2,6\}})

\]

\noindent and calculate the derivative according to your own email

address. When calculating the derivative, simplify all regular

expressions as much as possible by applying the

following 7 simplification rules:

\begin{center}

\begin{tabular}{l@{\hspace{2mm}}c@{\hspace{2mm}}ll}

$r \cdot \ZERO$ & $\mapsto$ & $\ZERO$\\ 

$\ZERO \cdot r$ & $\mapsto$ & $\ZERO$\\ 

$r \cdot \ONE$ & $\mapsto$ & $r$\\ 

$\ONE \cdot r$ & $\mapsto$ & $r$\\ 

$r + \ZERO$ & $\mapsto$ & $r$\\ 

$\ZERO + r$ & $\mapsto$ & $r$\\ 

$r + r$ & $\mapsto$ & $r$\\ 

\end{tabular}

\end{center}

\noindent Write down your simplified derivative in a readable

notation using parentheses where necessary. That means you

should use the infix notation $+$, $\cdot$, $^*$ and so on,

instead of raw code.\bigskip

\subsection*{Question 6}

Implement the simplification rules in your regular expression matcher.

Consider the regular expression $/ \cdot * \cdot

(\sim{}(\textit{ALL}^* \cdot * \cdot / \cdot \textit{ALL}^*)) \cdot *

\cdot /$ and decide whether the following four strings are matched by

this regular expression. Answer yes or no.

\begin{enumerate}

\item \texttt{"/**/"}

\item \texttt{"/*foobar*/"}

\item \texttt{"/*test*/test*/"}

\item \texttt{"/*test/*test*/"}

\end{enumerate}

\subsection*{Question 7}

Let $r_1$ be the regular expression $a\cdot a\cdot a$ and $r_2$ be

$(a^{\{19,19\}}) \cdot (a^?)$.\medskip

\noindent

Decide whether the following three

strings consisting of $a$s only can be matched by $(r_1^+)^+$.

Similarly test them with $(r_2^+)^+$. Again answer in all six cases

with yes or no. \medskip

\noindent

These are strings are meant to be entirely made up of $a$s. Be careful

when copy-and-pasting the strings so as to not forgetting any $a$ and

to not introducing any other character.

\begin{enumerate}

\setcounter{enumi}{4}

\item \texttt{"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}

\item \texttt{"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\ 

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\ 

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}

\item \texttt{"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\ 

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\ 

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}

\end{enumerate}

\end{document}

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