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\section*{Homework 4}

\begin{enumerate}
\item Why is every finite set of strings a regular language?

\item What is the language recognised by the regular expressions $(\varnothing^*)^*$.

\item If a regular expression $r$ does not contain any occurrence of $\varnothing$ 
is it possible for $L(r)$ to be empty?

\item Assume that $s^{-1}$ stands for the operation of reversing a
string $s$. Given the following \emph{reversing} function on regular 
expressions

\begin{center}
\begin{tabular}{r@{\hspace{1mm}}c@{\hspace{1mm}}l}
$rev(\varnothing)$   & $\dn$ & $\varnothing$\\
$rev(\epsilon)$         & $\dn$ & $\epsilon$\\
$rev(c)$                      & $\dn$ & $c$\\
$rev(r_1 + r_2)$        & $\dn$ & $rev(r_1) + rev(r_2)$\\
$rev(r_1 \cdot r_2)$  & $\dn$ & $rev(r_2) \cdot rev(r_1)$\\
$rev(r^*)$                   & $\dn$ & $rev(r)^*$\\
\end{tabular}
\end{center}


and the set

\begin{center}
$Rev\,A \dn \{s^{-1} \;|\; s \in A\}$
\end{center}

prove whether

\begin{center}
$L(rev(r)) = Rev (L(r))$
\end{center}

holds.

\item Give a regular expression over the alphabet $\{a,b\}$ recognising all strings 
that do not contain any substring $bb$ and end in $a$.

\item Assume the delimiters for comments are \texttt{$\slash$*} and \texttt{*$\slash$}.
Give a regular expression that can recognise comments
of the form 

\begin{center}
\texttt{$\slash$*~\ldots{}~*$\slash$} 
\end{center}

where the three dots stand for arbitrary characters, but not comment delimiters.
(Hint: You can assume you are already given a regular expression written \texttt{ALL},
that can recognise any character, and a regular expression \texttt{NOT} that recognises
the complement of a regular expression.)

\item Given the alphabet $\{a,b\}$. Draw the automaton that has two states, say  $q_0$ and $q_1$.
The starting state is $q_0$ and the final state is $q_1$. The transition
function is given by

\begin{center}
\begin{tabular}{l}
$(q_0, a) \rightarrow q_0$\\
$(q_0, b) \rightarrow q_1$\\
$(q_1, b) \rightarrow q_1$
\end{tabular}
\end{center}

What is the languages recognised by this automaton?

\item Give a deterministic finite automaton that can recognise 
the language $L(a^*\cdot b\cdot b^*)$. 


\item (Optional) The tokenizer in \texttt{regexp3.scala} takes as
argument a string and a list of rules. The result is a list of tokens. Improve this tokenizer so 
that it filters out all comments and whitespace from the result.

\item (Optional) Modify the tokenizer in \texttt{regexp2.scala} so that it
implements the \texttt{findAll} function. This function takes a regular
expressions and a string, and returns all substrings in this string that 
match the regular expression.
\end{enumerate}

% explain what is a context-free grammar and the language it generates 
%
%
% Define the language L(M) accepted by a deterministic finite automaton M.
%
%
% does (a + b)*b+ and (a*b+) + (b*b+) define the same language


\end{document}

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