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\begin{document}

\section*{Handout 3 (Automata)}

Every formal language course I know of bombards you first with
automata and then to a much, much smaller extend with regular
expressions. As you can see, this course is turned upside
down: regular expressions come first. The reason is that
regular expressions are easier to reason about and the notion
of derivatives, although already quite old, only became more
widely known rather recently. Still let us in this lecture
have a closer look at automata and their relation to regular
expressions. This will help us with understanding why the
regular expression matchers in Python and Ruby are so slow
with certain regular expressions. The central definition
is:\medskip 

\noindent 
A \emph{deterministic finite automaton} (DFA), say $A$, is
defined by a four-tuple written $A(Q, q_0, F, \delta)$ where

\begin{itemize}
\item $Q$ is a finite set of states,
\item $q_0 \in Q$ is the start state,
\item $F \subseteq Q$ are the accepting states, and
\item $\delta$ is the transition function.
\end{itemize}

\noindent The transition function determines how to
``transition'' from one state to the next state with respect
to a character. We have the assumption that these transition
functions do not need to be defined everywhere: so it can be
the case that given a character there is no next state, in
which case we need to raise a kind of ``failure exception''. A
typical example of a DFA is

\begin{center}
\begin{tikzpicture}[>=stealth',very thick,auto,
                    every state/.style={minimum size=0pt,
                    inner sep=2pt,draw=blue!50,very thick,
                    fill=blue!20},scale=2]
\node[state,initial]  (q_0)  {$q_0$};
\node[state] (q_1) [right=of q_0] {$q_1$};
\node[state] (q_2) [below right=of q_0] {$q_2$};
\node[state] (q_3) [right=of q_2] {$q_3$};
\node[state, accepting] (q_4) [right=of q_1] {$q_4$};
\path[->] (q_0) edge node [above]  {$a$} (q_1);
\path[->] (q_1) edge node [above]  {$a$} (q_4);
\path[->] (q_4) edge [loop right] node  {$a, b$} ();
\path[->] (q_3) edge node [right]  {$a$} (q_4);
\path[->] (q_2) edge node [above]  {$a$} (q_3);
\path[->] (q_1) edge node [right]  {$b$} (q_2);
\path[->] (q_0) edge node [above]  {$b$} (q_2);
\path[->] (q_2) edge [loop left] node  {$b$} ();
\path[->] (q_3) edge [bend left=95, looseness=1.3] node [below]  {$b$} (q_0);
\end{tikzpicture}
\end{center}

\noindent In this graphical notation, the accepting state
$q_4$ is indicated with double circles. Note that there can be
more than one accepting state. It is also possible that a DFA
has no accepting states at all, or that the starting state is
also an accepting state. In the case above the transition
function is defined everywhere and can be given as a table as
follows:

\[
\begin{array}{lcl}
(q_0, a) &\rightarrow& q_1\\
(q_0, b) &\rightarrow& q_2\\
(q_1, a) &\rightarrow& q_4\\
(q_1, b) &\rightarrow& q_2\\
(q_2, a) &\rightarrow& q_3\\
(q_2, b) &\rightarrow& q_2\\
(q_3, a) &\rightarrow& q_4\\
(q_3, b) &\rightarrow& q_0\\
(q_4, a) &\rightarrow& q_4\\
(q_4, b) &\rightarrow& q_4\\
\end{array}
\]

We need to define the notion of what language is accepted by
an automaton. For this we lift the transition function
$\delta$ from characters to strings as follows:

\[
\begin{array}{lcl}
\hat{\delta}(q, [])        & \dn & q\\
\hat{\delta}(q, c\!::\!s) & \dn & \hat{\delta}(\delta(q, c), s)\\
\end{array}
\]

\noindent This lifted transition function is often called
``delta-hat''. Given a string, we start in the starting state
and take the first character of the string, follow to the next
sate, then take the second character and so on. Once the
string is exhausted and we end up in an accepting state, then
this string is accepted by the automaton. Otherwise it is not
accepted. So $s$ is in the \emph{language accepted by the
automaton} $A(Q, q_0, F, \delta)$ iff

\[
\hat{\delta}(q_0, s) \in F 
\]

\noindent I let you think about a definition that describes
the set of strings accepted by an automaton.
  

While with DFAs it will always be clear that given a character
what the next state is (potentially none), it will be useful
to relax this restriction. That means we have several
potential successor states. We even allow ``silent
transitions'', also called epsilon-transitions. They allow us
to go from one state to the next without having a character
consumed. We label such silent transition with the letter
$\epsilon$. The resulting construction is called a
\emph{non-deterministic finite automaton} (NFA) given also as
a four-tuple $A(Q, q_0, F, \rho)$ where

\begin{itemize}
\item $Q$ is a finite set of states
\item $q_0$ is a start state
\item $F$ are some accepting states with $F \subseteq Q$, and
\item $\rho$ is a transition relation.
\end{itemize}

\noindent
Two typical examples of NFAs are
\begin{center}
\begin{tabular}[t]{c@{\hspace{9mm}}c}
\begin{tikzpicture}[scale=0.7,>=stealth',very thick,
                             every state/.style={minimum size=0pt,draw=blue!50,very thick,fill=blue!20},]
\node[state,initial]  (q_0)  {$q_0$};
\node[state] (q_1) [above=of q_0] {$q_1$};
\node[state, accepting] (q_2) [below=of q_0] {$q_2$};
\path[->] (q_0) edge node [left]  {$\epsilon$} (q_1);
\path[->] (q_0) edge node [left]  {$\epsilon$} (q_2);
\path[->] (q_0) edge [loop right] node  {$a$} ();
\path[->] (q_1) edge [loop above] node  {$a$} ();
\path[->] (q_2) edge [loop below] node  {$b$} ();
\end{tikzpicture} &

\raisebox{20mm}{
\begin{tikzpicture}[scale=0.7,>=stealth',very thick,
                             every state/.style={minimum size=0pt,draw=blue!50,very thick,fill=blue!20},]
\node[state,initial]  (r_1)  {$r_1$};
\node[state] (r_2) [above=of r_1] {$r_2$};
\node[state, accepting] (r_3) [right=of r_1] {$r_3$};
\path[->] (r_1) edge node [below]  {$b$} (r_3);
\path[->] (r_2) edge [bend left] node [above]  {$a$} (r_3);
\path[->] (r_1) edge [bend left] node  [left] {$\epsilon$} (r_2);
\path[->] (r_2) edge [bend left] node  [right] {$a$} (r_1);
\end{tikzpicture}}
\end{tabular}
\end{center}

\noindent There are, however, a number of points you should
note. Every DFA is a NFA, but not vice versa. The $\rho$ in
NFAs is a transition \emph{relation} (DFAs have a transition
function). The difference between a function and a relation is
that a function has always a single output, while a relation
gives, roughly speaking, several outputs. Look at the NFA on
the right-hand side above: if you are currently in the state
$r_2$ and you read a character $a$, then you can transition to
either $r_1$ \emph{or} $r_3$. Which route you take is not
determined. This means if we need to decide whether a string
is accepted by a NFA, we might have to explore all
possibilities. Also there is the special silent transition in
NFAs. As mentioned already this transition means you do not
have to ``consume'' any part of the input string, but
``silently'' change to a different state. In the left picture,
for example, if you are in the starting state, you can 
silently move either to $q_1$ or $q_2$.


\subsubsection*{Thompson Construction}

The reason for introducing NFAs is that there is a relatively
simple (recursive) translation of regular expressions into
NFAs. Consider the simple regular expressions $\varnothing$,
$\epsilon$ and $c$. They can be translated as follows:

\begin{center}
\begin{tabular}[t]{l@{\hspace{10mm}}l}
\raisebox{1mm}{$\varnothing$} & 
\begin{tikzpicture}[scale=0.7,>=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node[state, initial]  (q_0)  {$\mbox{}$};
\end{tikzpicture}\\\\
\raisebox{1mm}{$\epsilon$} & 
\begin{tikzpicture}[scale=0.7,>=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node[state, initial, accepting]  (q_0)  {$\mbox{}$};
\end{tikzpicture}\\\\
\raisebox{2mm}{$c$} & 
\begin{tikzpicture}[scale=0.7,>=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node[state, initial]  (q_0)  {$\mbox{}$};
\node[state, accepting]  (q_1)  [right=of q_0] {$\mbox{}$};
\path[->] (q_0) edge node [below]  {$c$} (q_1);
\end{tikzpicture}\\\\
\end{tabular}
\end{center}

\noindent The case for the sequence regular expression $r_1
\cdot r_2$ is as follows: We are given by recursion two
automata representing $r_1$ and $r_2$ respectively. 

\begin{center}
\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node[state, initial]  (q_0)  {$\mbox{}$};
\node (r_1)  [right=of q_0] {$\ldots$};
\node[state, accepting]  (t_1)  [right=of r_1] {$\mbox{}$};
\node[state, accepting]  (t_2)  [above=of t_1] {$\mbox{}$};
\node[state, accepting]  (t_3)  [below=of t_1] {$\mbox{}$};
\node[state, initial]  (a_0)  [right=2.5cm of t_1] {$\mbox{}$};
\node (b_1)  [right=of a_0] {$\ldots$};
\node[state, accepting]  (c_1)  [right=of b_1] {$\mbox{}$};
\node[state, accepting]  (c_2)  [above=of c_1] {$\mbox{}$};
\node[state, accepting]  (c_3)  [below=of c_1] {$\mbox{}$};
\begin{pgfonlayer}{background}
\node (1) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (q_0) (r_1) (t_1) (t_2) (t_3)] {};
\node (2) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (a_0) (b_1) (c_1) (c_2) (c_3)] {};
\node [yshift=2mm] at (1.north) {$r_1$};
\node [yshift=2mm] at (2.north) {$r_2$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent The first automaton has some accepting states. We
obtain an automaton for $r_1\cdot r_2$ by connecting these
accepting states with $\epsilon$-transitions to the starting
state of the second automaton. By doing so we make them
non-accepting like so:

\begin{center}
\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node[state, initial]  (q_0)  {$\mbox{}$};
\node (r_1)  [right=of q_0] {$\ldots$};
\node[state]  (t_1)  [right=of r_1] {$\mbox{}$};
\node[state]  (t_2)  [above=of t_1] {$\mbox{}$};
\node[state]  (t_3)  [below=of t_1] {$\mbox{}$};
\node[state]  (a_0)  [right=2.5cm of t_1] {$\mbox{}$};
\node (b_1)  [right=of a_0] {$\ldots$};
\node[state, accepting]  (c_1)  [right=of b_1] {$\mbox{}$};
\node[state, accepting]  (c_2)  [above=of c_1] {$\mbox{}$};
\node[state, accepting]  (c_3)  [below=of c_1] {$\mbox{}$};
\path[->] (t_1) edge node [above, pos=0.3]  {$\epsilon$} (a_0);
\path[->] (t_2) edge node [above]  {$\epsilon$} (a_0);
\path[->] (t_3) edge node [below]  {$\epsilon$} (a_0);

\begin{pgfonlayer}{background}
\node (3) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (q_0) (c_1) (c_2) (c_3)] {};
\node [yshift=2mm] at (3.north) {$r_1\cdot r_2$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent The case for the choice regular expression $r_1 +
r_2$ is slightly different: We are given by recursion two
automata representing $r_1$ and $r_2$ respectively. 

\begin{center}
\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node at (0,0)  (1)  {$\mbox{}$};
\node[state, initial]  (2)  [above right=16mm of 1] {$\mbox{}$};
\node[state, initial]  (3)  [below right=16mm of 1] {$\mbox{}$};

\node (a)  [right=of 2] {$\ldots$};
\node[state, accepting]  (a1)  [right=of a] {$\mbox{}$};
\node[state, accepting]  (a2)  [above=of a1] {$\mbox{}$};
\node[state, accepting]  (a3)  [below=of a1] {$\mbox{}$};

\node (b)  [right=of 3] {$\ldots$};
\node[state, accepting]  (b1)  [right=of b] {$\mbox{}$};
\node[state, accepting]  (b2)  [above=of b1] {$\mbox{}$};
\node[state, accepting]  (b3)  [below=of b1] {$\mbox{}$};
\begin{pgfonlayer}{background}
\node (1) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (2) (a1) (a2) (a3)] {};
\node (2) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (3) (b1) (b2) (b3)] {};
\node [yshift=3mm] at (1.north) {$r_1$};
\node [yshift=3mm] at (2.north) {$r_2$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent Each automaton has a single start state and
potentially several accepting states. We obtain a NFA for the
regular expression $r_1 + r_2$ by introducing a new starting
state and connecting it with an $\epsilon$-transition to the
two starting states above, like so

\begin{center}
\hspace{2cm}\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node at (0,0) [state, initial]  (1)  {$\mbox{}$};
\node[state]  (2)  [above right=16mm of 1] {$\mbox{}$};
\node[state]  (3)  [below right=16mm of 1] {$\mbox{}$};

\node (a)  [right=of 2] {$\ldots$};
\node[state, accepting]  (a1)  [right=of a] {$\mbox{}$};
\node[state, accepting]  (a2)  [above=of a1] {$\mbox{}$};
\node[state, accepting]  (a3)  [below=of a1] {$\mbox{}$};

\node (b)  [right=of 3] {$\ldots$};
\node[state, accepting]  (b1)  [right=of b] {$\mbox{}$};
\node[state, accepting]  (b2)  [above=of b1] {$\mbox{}$};
\node[state, accepting]  (b3)  [below=of b1] {$\mbox{}$};

\path[->] (1) edge node [above]  {$\epsilon$} (2);
\path[->] (1) edge node [below]  {$\epsilon$} (3);

\begin{pgfonlayer}{background}
\node (3) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (1) (a2) (a3) (b2) (b3)] {};
\node [yshift=3mm] at (3.north) {$r_1+ r_2$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent 
Finally for the $*$-case we have an automaton for $r$

\begin{center}
\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node at (0,0)  (1)  {$\mbox{}$};
\node[state, initial]  (2)  [right=16mm of 1] {$\mbox{}$};
\node (a)  [right=of 2] {$\ldots$};
\node[state, accepting]  (a1)  [right=of a] {$\mbox{}$};
\node[state, accepting]  (a2)  [above=of a1] {$\mbox{}$};
\node[state, accepting]  (a3)  [below=of a1] {$\mbox{}$};
\begin{pgfonlayer}{background}
\node (1) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (2) (a1) (a2) (a3)] {};
\node [yshift=3mm] at (1.north) {$r$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent and connect its accepting states to a new starting
state via $\epsilon$-transitions. This new starting state is
also an accepting state, because $r^*$ can recognise the
empty string. This gives the following automaton for $r^*$:

\begin{center}
\begin{tikzpicture}[node distance=3mm,
                             >=stealth',very thick, every state/.style={minimum size=3pt,draw=blue!50,very thick,fill=blue!20},]
\node at (0,0) [state, initial,accepting]  (1)  {$\mbox{}$};
\node[state]  (2)  [right=16mm of 1] {$\mbox{}$};
\node (a)  [right=of 2] {$\ldots$};
\node[state]  (a1)  [right=of a] {$\mbox{}$};
\node[state]  (a2)  [above=of a1] {$\mbox{}$};
\node[state]  (a3)  [below=of a1] {$\mbox{}$};
\path[->] (1) edge node [above]  {$\epsilon$} (2);
\path[->] (a1) edge [bend left=45] node [above]  {$\epsilon$} (1);
\path[->] (a2) edge [bend right] node [below]  {$\epsilon$} (1);
\path[->] (a3) edge [bend left=45] node [below]  {$\epsilon$} (1);
\begin{pgfonlayer}{background}
\node (2) [rounded corners, inner sep=1mm, thick, draw=black!60, fill=black!20, fit= (1) (a2) (a3)] {};
\node [yshift=3mm] at (2.north) {$r^*$};
\end{pgfonlayer}
\end{tikzpicture}
\end{center}

\noindent This construction of a NFA from a regular expression
was invented by Ken Thompson in 1968.


\subsubsection*{Subset Construction}

What is interesting is that for every NFA we can find a DFA
which recognises the same language. This can, for example, be
done by the \emph{subset construction}. Consider again the NFA
below on the left, consisting of nodes labeled $0$, $1$ and $2$. 

\begin{center}
\begin{tabular}{c@{\hspace{10mm}}c}
\begin{tikzpicture}[scale=0.7,>=stealth',very thick,
                    every state/.style={minimum size=0pt,
                    draw=blue!50,very thick,fill=blue!20},
                    baseline=0mm]
\node[state,initial]  (q_0)  {$0$};
\node[state] (q_1) [above=of q_0] {$1$};
\node[state, accepting] (q_2) [below=of q_0] {$2$};
\path[->] (q_0) edge node [left]  {$\epsilon$} (q_1);
\path[->] (q_0) edge node [left]  {$\epsilon$} (q_2);
\path[->] (q_0) edge [loop right] node  {$a$} ();
\path[->] (q_1) edge [loop above] node  {$a$} ();
\path[->] (q_2) edge [loop below] node  {$b$} ();
\end{tikzpicture}
&
\begin{tabular}{r|cl}
nodes & $a$ & $b$\\
\hline
$\{\}\phantom{\star}$ & $\{\}$ & $\{\}$\\
$\{0\}\phantom{\star}$       & $\{0,1,2\}$   & $\{2\}$\\
$\{1\}\phantom{\star}$       & $\{1\}$       & $\{\}$\\
$\{2\}\star$  & $\{\}$ & $\{2\}$\\
$\{0,1\}\phantom{\star}$     & $\{0,1,2\}$   & $\{2\}$\\
$\{0,2\}\star$ & $\{0,1,2\}$   & $\{2\}$\\
$\{1,2\}\star$ & $\{1\}$       & $\{2\}$\\
s: $\{0,1,2\}\star$ & $\{0,1,2\}$ & $\{2\}$\\
\end{tabular}
\end{tabular}
\end{center}

\noindent The nodes of the DFA are given by calculating all
subsets of the set of nodes of the NFA (seen in the nodes
column on the right). The table shows the transition function
for the DFA. The first row states that $\{\}$ is the
sink node which has transitions for $a$ and $b$ to itself.
The next three lines are calculated as follows: 

\begin{itemize}
\item suppose you calculate the entry for the transition for
      $a$ and the node $\{0\}$
\item start from the node $0$ in the NFA
\item do as many $\epsilon$-transition as you can obtaining a
      set of nodes, in this case $\{0,1,2\}$
\item filter out all notes that do not allow an $a$-transition
      from this set, this excludes $2$ which does not permit a
      $a$-transition
\item from the remaining set, do as many $\epsilon$-transition
      as you can, this yields again $\{0,1,2\}$     
\item the resulting set specifies the transition from $\{0\}$
      when given an $a$ 
\end{itemize}

\noindent So the transition from the state $\{0\}$ reading an
$a$ goes to the state $\{0,1,2\}$. Similarly for the other
entries in the rows for $\{0\}$, $\{1\}$ and $\{2\}$. The
other rows are calculated by just taking the union of the
single node entries. For example for $a$ and $\{0,1\}$ we need
to take the union of $\{0,1,2\}$ (for $0$) and $\{1\}$ (for
$1$). The starting state of the DFA can be calculated from the
starting state of the NFA, that is $0$, and then do as many
$\epsilon$-transitions as possible. This gives $\{0,1,2\}$
which is the starting state of the DFA. The terminal states in
the DFA are given by all sets that contain a $2$, which is the
terminal state of the NFA. This completes the subset
construction. So the corresponding DFA to the NFA from 
above is

\begin{center}
\begin{tikzpicture}[scale=0.7,>=stealth',very thick,
                    every state/.style={minimum size=0pt,
                    draw=blue!50,very thick,fill=blue!20},
                    baseline=0mm]
\node[state,initial,accepting]  (q012)  {$0,1,2$};
\node[state,accepting] (q02) [right=of q012] {$0,2$};
\node[state] (q01) [above=of q02] {$0,1$};
\node[state,accepting] (q12) [below=of q02] {$1,2$};
\node[state] (q0) [right=2cm of q01] {$0$};
\node[state] (q1) [right=2.5cm of q02] {$1$};
\node[state,accepting] (q2) [right=1.5cm of q12] {$2$};
\node[state] (qn) [right=of q1] {$\{\}$};

\path[->] (q012) edge [loop below] node  {$a$} ();
\path[->] (q012) edge node [above]  {$b$} (q2);
\path[->] (q12) edge [bend left] node [below,pos=0.4]  {$a$} (q1);
\path[->] (q12) edge node [below]  {$b$} (q2);
\path[->] (q02) edge node [above]  {$a$} (q012);
\path[->] (q02) edge [bend left] node [above, pos=0.8]  {$b$} (q2);
\path[->] (q01) edge node [below]  {$a$} (q012);
\path[->] (q01) edge [bend left] node [above]  {$b$} (q2);
\path[->] (q0) edge node [below]  {$a$} (q012);
\path[->] (q0) edge node [right, pos=0.2]  {$b$} (q2);
\path[->] (q1) edge [loop above] node  {$a$} ();
\path[->] (q1) edge node [above]  {$b$} (qn);
\path[->] (q2) edge [loop right] node  {$b$} ();
\path[->] (q2) edge node [below]  {$a$} (qn);
\path[->] (qn) edge [loop above] node  {$a,b$} ();
\end{tikzpicture}
\end{center}



There are two points to note: One is that very often the
resulting DFA contains a number of ``dead'' nodes that are
never reachable from the starting state. For example
there is no way to reach node $\{0,2\}$ from the starting
state $\{0,1,2\}$. I let you find the other dead states.
In effect the DFA in this example is not a minimal DFA. Such
dead nodes can be safely removed without changing the language
that is recognised by the DFA. Another point is that in some
cases, however, the subset construction produces a DFA that
does \emph{not} contain any dead nodes\ldots{}that means it
calculates a minimal DFA. Which in turn means that in some
cases the number of nodes by going from NFAs to DFAs
exponentially increases, namely by $2^n$ (which is the number
of subsets you can form for $n$ nodes). 

Removing all the dead states in the automaton above,
gives a much more legible automaton, namely

\begin{center}
\begin{tikzpicture}[scale=0.7,>=stealth',very thick,
                    every state/.style={minimum size=0pt,
                    draw=blue!50,very thick,fill=blue!20},
                    baseline=0mm]
\node[state,initial,accepting]  (q012)  {$0,1,2$};
\node[state,accepting] (q2) [right=of q012] {$2$};
\node[state] (qn) [right=of q2] {$\{\}$};

\path[->] (q012) edge [loop below] node  {$a$} ();
\path[->] (q012) edge node [above]  {$b$} (q2);
\path[->] (q2) edge [loop below] node  {$b$} ();
\path[->] (q2) edge node [below]  {$a$} (qn);
\path[->] (qn) edge [loop above] node  {$a,b$} ();
\end{tikzpicture}
\end{center}

\noindent Now the big question is whether this DFA
can recognise the same language as the NFA we started with.
I let you ponder about this question.

\subsubsection*{Brzozowski's Method}

As said before, we can also go into the other direction---from
DFAs to regular expressions. Brzozowski's method calculates
a regular expression using familiar transformations for
solving equational systems. Consider the DFA:

\begin{center}
\begin{tikzpicture}[scale=1.5,>=stealth',very thick,auto,
                    every state/.style={minimum size=0pt,
                    inner sep=2pt,draw=blue!50,very thick,
                    fill=blue!20}]
  \node[state, initial]        (q0) at ( 0,1) {$q_0$};
  \node[state]                    (q1) at ( 1,1) {$q_1$};
  \node[state, accepting] (q2) at ( 2,1) {$q_2$};
  \path[->] (q0) edge[bend left] node[above] {$a$} (q1)
            (q1) edge[bend left] node[above] {$b$} (q0)
            (q2) edge[bend left=50] node[below] {$b$} (q0)
            (q1) edge node[above] {$a$} (q2)
            (q2) edge [loop right] node {$a$} ()
            (q0) edge [loop below] node {$b$} ();
\end{tikzpicture}
\end{center}

\noindent for which we can set up the following equational
system

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,b + q_1\,b +  q_2\,b\\
q_1 & = & q_0\,a\\
q_2 & = & q_1\,a + q_2\,a
\end{eqnarray}

\noindent There is an equation for each node in the DFA. Let
us have a look how the right-hand sides of the equations are
constructed. First have a look at the second equation: the
left-hand side is $q_1$ and the right-hand side $q_0\,a$. The
right-hand side is essentially all possible ways how to end up
in $q_1$. There is only one incoming edge from $q_0$ consuming
an $a$.  Therefore the right hand side is this
state followed by character---in this case $q_0\,a$. Now lets
have a look at the third equation: there are two incoming
edges for $q_2$. Therefore we have two terms, namely $q_1\,a$ and
$q_2\,a$. These terms are separated by $+$. The first states
that if in state $q_1$ consuming an $a$ will bring you to
$q_2$, and the secont that being in $q_2$ and consuming an $a$
will make you stay in $q_2$. The right-hand side of the
first equation is constructed similarly: there are three 
incoming edges, therefore there are three terms. There is
one exception in that we also ``add'' $\epsilon$ to the
first equation, because it corresponds to the starting state
in the DFA.

Having constructed the equational system, the question is
how to solve it? Remarkably the rules are very similar to
solving usual linear equational systems. For example the
second equation does not contain the variable $q_1$ on the
right-hand side of the equation. We can therefore eliminate 
$q_1$ from the system by just substituting this equation
into the other two. This gives

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,b + q_0\,a\,b +  q_2\,b\\
q_2 & = & q_0\,a\,a + q_2\,a
\end{eqnarray}

\noindent where in Equation (4) we have two occurences
of $q_0$. Like the laws about $+$ and $\cdot$, we can simplify 
Equation (4) to obtain the following two equations:

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,(b + a\,b) +  q_2\,b\\
q_2 & = & q_0\,a\,a + q_2\,a
\end{eqnarray}
 
\noindent Unfortunately we cannot make any more progress with
substituting equations, because both (6) and (7) contain the
variable on the left-hand side also on the right-hand side.
Here we need to now use a law that is different from the usual
laws about linear equations. It is called \emph{Arden's rule}.
It states that if an equation is of the form $q = q\,r + s$
then it can be transformed to $q = s\, r^*$. Since we can
assume $+$ is symmetric, Equation (7) is of that form: $s$ is
$q_0\,a\,a$ and $r$ is $a$. That means we can transform
(7) to obtain the two new equations

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,(b + a\,b) +  q_2\,b\\
q_2 & = & q_0\,a\,a\,(a^*)
\end{eqnarray}

\noindent Now again we can substitute the second equation into
the first in order to eliminate the variable $q_2$.

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,(b + a\,b) +  q_0\,a\,a\,(a^*)\,b
\end{eqnarray}

\noindent Pulling $q_0$ out as a single factor gives:

\begin{eqnarray}
q_0 & = & \epsilon + q_0\,(b + a\,b + a\,a\,(a^*)\,b)
\end{eqnarray}

\noindent This equation is again of the form so that we can
apply Arden's rule ($r$ is $b + a\,b + a\,a\,(a^*)\,b$ and $s$
is $\epsilon$). This gives as solution for $q_0$ the following
regular expression:

\begin{eqnarray}
q_0 & = & \epsilon\,(b + a\,b + a\,a\,(a^*)\,b)^*
\end{eqnarray}

\noindent Since this is a regular expression, we can simplify
away the $\epsilon$ to obtain the slightly simpler regular
expression

\begin{eqnarray}
q_0 & = & (b + a\,b + a\,a\,(a^*)\,b)^*
\end{eqnarray}

\noindent 
Now we can unwind this process and obtain the solutions
for the other equations. This gives:

\begin{eqnarray}
q_0 & = & (b + a\,b + a\,a\,(a^*)\,b)^*\\
q_1 & = & (b + a\,b + a\,a\,(a^*)\,b)^*\,a\\
q_2 & = & (b + a\,b + a\,a\,(a^*)\,b)^*\,a\,a\,(a)^*
\end{eqnarray}

\noindent Finally, we only need to ``add'' up the equations
which correspond to a terminal state. In our running example,
this is just $q_2$. Consequently, a regular expression
that recognises the same language as the automaton is

\[
(b + a\,b + a\,a\,(a^*)\,b)^*\,a\,a\,(a)^*
\]

\noindent You can somewhat crosscheck your solution
by taking a string the regular expression can match and
and see whether it can be matched by the automaton.
One string for example is $aaa$ and \emph{voila} this 
string is also matched by the automaton.

We should prove that Brzozowski's method really produces
an equivalent  regular expression for the automaton. But
for the purposes of this module, we omit this.

\subsubsection*{Automata Minimization}

As seen in the subset construction, the translation 
of an NFA to a DFA can result in a rather ``inefficient'' 
DFA. Meaning there are states that are not needed. A
DFA can be \emph{minimised} by the following algorithm:

\begin{enumerate}
\item Take all pairs $(q, p)$ with $q \not= p$
\item Mark all pairs that accepting and non-accepting states
\item For all unmarked pairs $(q, p)$ and all characters $c$
      test whether 
      
      \begin{center} 
      $(\delta(q, c), \delta(p,c))$ 
      \end{center} 
      
      are marked. If there is one, then also mark $(q, p)$.
\item Repeat last step until no change.
\item All unmarked pairs can be merged.
\end{enumerate}

\noindent To illustrate this algorithm, consider the following 
DFA.

\begin{center}
\begin{tikzpicture}[>=stealth',very thick,auto,
                    every state/.style={minimum size=0pt,
                    inner sep=2pt,draw=blue!50,very thick,
                    fill=blue!20}]
\node[state,initial]  (q_0)  {$q_0$};
\node[state] (q_1) [right=of q_0] {$q_1$};
\node[state] (q_2) [below right=of q_0] {$q_2$};
\node[state] (q_3) [right=of q_2] {$q_3$};
\node[state, accepting] (q_4) [right=of q_1] {$q_4$};
\path[->] (q_0) edge node [above]  {$a$} (q_1);
\path[->] (q_1) edge node [above]  {$a$} (q_4);
\path[->] (q_4) edge [loop right] node  {$a, b$} ();
\path[->] (q_3) edge node [right]  {$a$} (q_4);
\path[->] (q_2) edge node [above]  {$a$} (q_3);
\path[->] (q_1) edge node [right]  {$b$} (q_2);
\path[->] (q_0) edge node [above]  {$b$} (q_2);
\path[->] (q_2) edge [loop left] node  {$b$} ();
\path[->] (q_3) edge [bend left=95, looseness=1.3] node 
  [below]  {$b$} (q_0);
\end{tikzpicture}
\end{center}

\noindent In Step 1 and 2 we consider essentially a triangle
of the form

\begin{center}
\begin{tikzpicture}[scale=0.6,line width=0.8mm]
\draw (0,0) -- (4,0);
\draw (0,1) -- (4,1);
\draw (0,2) -- (3,2);
\draw (0,3) -- (2,3);
\draw (0,4) -- (1,4);

\draw (0,0) -- (0, 4);
\draw (1,0) -- (1, 4);
\draw (2,0) -- (2, 3);
\draw (3,0) -- (3, 2);
\draw (4,0) -- (4, 1);

\draw (0.5,-0.5) node {$q_0$}; 
\draw (1.5,-0.5) node {$q_1$}; 
\draw (2.5,-0.5) node {$q_2$}; 
\draw (3.5,-0.5) node {$q_3$};
 
\draw (-0.5, 3.5) node {$q_1$}; 
\draw (-0.5, 2.5) node {$q_2$}; 
\draw (-0.5, 1.5) node {$q_3$}; 
\draw (-0.5, 0.5) node {$q_4$}; 

\draw (0.5,0.5) node {\large$\star$}; 
\draw (1.5,0.5) node {\large$\star$}; 
\draw (2.5,0.5) node {\large$\star$}; 
\draw (3.5,0.5) node {\large$\star$};
\end{tikzpicture}
\end{center}

\noindent where the lower row is filled with stars, because in
the corresponding pairs there is always one state that is
accepting ($q_4$) and a state that is non-accepting (the other
states).

Now in Step 3 we need to fill in more stars according whether 
one of the next-state pairs are marked. We have to do this 
for every unmarked field until there is no change anymore.
This gives the triangle

\begin{center}
\begin{tikzpicture}[scale=0.6,line width=0.8mm]
\draw (0,0) -- (4,0);
\draw (0,1) -- (4,1);
\draw (0,2) -- (3,2);
\draw (0,3) -- (2,3);
\draw (0,4) -- (1,4);

\draw (0,0) -- (0, 4);
\draw (1,0) -- (1, 4);
\draw (2,0) -- (2, 3);
\draw (3,0) -- (3, 2);
\draw (4,0) -- (4, 1);

\draw (0.5,-0.5) node {$q_0$}; 
\draw (1.5,-0.5) node {$q_1$}; 
\draw (2.5,-0.5) node {$q_2$}; 
\draw (3.5,-0.5) node {$q_3$};
 
\draw (-0.5, 3.5) node {$q_1$}; 
\draw (-0.5, 2.5) node {$q_2$}; 
\draw (-0.5, 1.5) node {$q_3$}; 
\draw (-0.5, 0.5) node {$q_4$}; 

\draw (0.5,0.5) node {\large$\star$}; 
\draw (1.5,0.5) node {\large$\star$}; 
\draw (2.5,0.5) node {\large$\star$}; 
\draw (3.5,0.5) node {\large$\star$};
\draw (0.5,1.5) node {\large$\star$}; 
\draw (2.5,1.5) node {\large$\star$}; 
\draw (0.5,3.5) node {\large$\star$}; 
\draw (1.5,2.5) node {\large$\star$}; 
\end{tikzpicture}
\end{center}

\noindent which means states $q_0$ and $q_2$, as well as $q_1$
and $q_3$ can be merged. This gives the following minimal DFA

\begin{center}
\begin{tikzpicture}[>=stealth',very thick,auto,
                    every state/.style={minimum size=0pt,
                    inner sep=2pt,draw=blue!50,very thick,
                    fill=blue!20}]
\node[state,initial]  (q_02)  {$q_{0, 2}$};
\node[state] (q_13) [right=of q_02] {$q_{1, 3}$};
\node[state, accepting] (q_4) [right=of q_13] 
  {$q_{4\phantom{,0}}$};
\path[->] (q_02) edge [bend left] node [above]  {$a$} (q_13);
\path[->] (q_13) edge [bend left] node [below]  {$b$} (q_02);
\path[->] (q_02) edge [loop below] node  {$b$} ();
\path[->] (q_13) edge node [above]  {$a$} (q_4);
\path[->] (q_4) edge [loop above] node  {$a, b$} ();
\end{tikzpicture}
\end{center}

\subsubsection*{Regular Languages}

Given the constructions in the previous sections we obtain 
the following overall picture:

\begin{center}
\begin{tikzpicture}
\node (rexp)  {\bf Regexps};
\node (nfa) [right=of rexp] {\bf NFAs};
\node (dfa) [right=of nfa] {\bf DFAs};
\node (mdfa) [right=of dfa] {\bf\begin{tabular}{c}minimal\\ DFAs\end{tabular}};
\path[->,line width=1mm] (rexp) edge node [above=4mm, black] {\begin{tabular}{c@{\hspace{9mm}}}Thompson's\\[-1mm] construction\end{tabular}} (nfa);
\path[->,line width=1mm] (nfa) edge node [above=4mm, black] {\begin{tabular}{c}subset\\[-1mm] construction\end{tabular}}(dfa);
\path[->,line width=1mm] (dfa) edge node [below=5mm, black] {minimisation} (mdfa);
\path[->,line width=1mm] (dfa) edge [bend left=45] node [below] {\begin{tabular}{l}Brzozowski's\\ method\end{tabular}} (rexp);
\end{tikzpicture}
\end{center}

\noindent By going from regular expressions over NFAs to DFAs,
we can always ensure that for every regular expression there
exists a NFA and a DFA that can recognise the same language.
Although we did not prove this fact. Similarly by going from
DFAs to regular expressions, we can make sure for every DFA 
there exists a regular expression that can recognise the same
language. Again we did not prove this fact. 

The interesting conclusion is that automata and regular 
expressions can recognise the same set of languages:

\begin{quote} A language is \emph{regular} iff there exists a
regular expression that recognises all its strings.
\end{quote}

\noindent or equivalently 

\begin{quote} A language is \emph{regular} iff there exists an
automaton that recognises all its strings.
\end{quote}

\noindent So for deciding whether a string is recognised by a
regular expression, we could use our algorithm based on
derivatives or NFAs or DFAs. But let us quickly look at what
the differences mean in computational terms. Translating a
regular expression into a NFA gives us an automaton that has
$O(n)$ nodes---that means the size of the NFA grows linearly
with the size of the regular expression. The problem with NFAs
is that the problem of deciding whether a string is accepted
or not is computationally not cheap. Remember with NFAs we
have potentially many next states even for the same input and
also have the silent $\epsilon$-transitions. If we want to
find a path from the starting state of an NFA to an accepting
state, we need to consider all possibilities. In Ruby and
Python this is done by a depth-first search, which in turn
means that if a ``wrong'' choice is made, the algorithm has to
backtrack and thus explore all potential candidates. This is
exactly the reason why Ruby and Python are so slow for evil
regular expressions. An alternative to the potentially slow
depth-first search is to explore the search space in a
breadth-first fashion, but this might incur a big memory
penalty.  

To avoid the problems with NFAs, we can translate them 
into DFAs. With DFAs the problem of deciding whether a
string is recognised or not is much simpler, because in
each state it is completely determined what the next
state will be for a given input. So no search is needed.
The problem with this is that the translation to DFAs
can explode exponentially the number of states. Therefore when 
this route is taken, we definitely need to minimise the
resulting DFAs in order to have an acceptable memory 
and runtime behaviour. But remember the subset construction
in the worst case explodes the number of states by $2^n$.
Effectively also the translation to DFAs can incur a big
runtime penalty.

But this does not mean that everything is bad with automata.
Recall the problem of finding a regular expressions for the
language that is \emph{not} recognised by a regular
expression. In our implementation we added explicitly such a
regular expressions because they are useful for recognising
comments. But in principle we did not need to. The argument
for this is as follows: take a regular expression, translate
it into a NFA and then a DFA that both recognise the same
language. Once you have the DFA it is very easy to construct
the automaton for the language not recognised by an DFA. If
the DFA is completed (this is important!), then you just need
to exchange the accepting and non-accepting states. You can
then translate this DFA back into a regular expression and
that will be the regular expression that can match all strings
the original regular expression could \emph{not} match.

It is also interesting that not all languages are regular. The
most well-known example of a language that is not regular
consists of all the strings of the form

\[a^n\,b^n\]

\noindent meaning strings that have the same number of $a$s
and $b$s. You can try, but you cannot find a regular
expression for this language and also not an automaton. One
can actually prove that there is no regular expression nor
automaton for this language, but again that would lead us too
far afield for what we want to do in this module.

\section*{Further Reading}

Compare what a ``human expert'' would create as automaton for the
regular expression $a (b + c)^*$ and what the Thomson
algorithm generates.

%http://www.inf.ed.ac.uk/teaching/courses/ct/
\end{document}

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