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\begin{document}

\section*{Handout 6}

While regular expressions are very useful for lexing and for recognising
many patterns (like email addresses), they have their limitations. For
example there is no regular expression that can recognise the language 
$a^nb^n$. Another example is the language of well-parenthesised 
expressions.  In languages like Lisp, which use parentheses rather
extensively, it might be of interest whether the following two expressions
are well-parenthesised (the left one is, the right one is not):

\begin{center}
$(((()()))())$  \hspace{10mm} $(((()()))()))$
\end{center}

In order to solve such recognition problems, we need more powerful 
techniques than regular expressions. We will in particular look at \emph{context-free
languages}. They include the regular languages as the picture below shows:


\begin{center}
\begin{tikzpicture}
[rect/.style={draw=black!50, top color=white,bottom color=black!20, rectangle, very thick, rounded corners}]

\draw (0,0) node [rect, text depth=30mm, text width=46mm] {all languages};
\draw (0,-0.4) node [rect, text depth=20mm, text width=44mm] {decidable languages};
\draw (0,-0.65) node [rect, text depth=13mm] {context sensitive languages};
\draw (0,-0.84) node [rect, text depth=7mm, text width=35mm] {context-free languages};
\draw (0,-1.05) node [rect] {regular languages};
\end{tikzpicture}
\end{center}

\noindent
Context-free languages play an important role in `day-to-day' text processing and in
programming languages. Context-free languages are usually specified by grammars.
For example a grammar for well-parenthesised  expressions is

\begin{center}
$P \;\;\rightarrow\;\; ( \cdot  P \cdot ) \cdot P \;|\; \epsilon$
\end{center}
 
\noindent
In general grammars consist of finitely many rules built up from terminal symbols (usually lower-case letters)
and non-terminal symbols (upper-case letters).  Rules have the shape

\begin{center}
$NT \;\;\rightarrow\;\; \textit{rhs}$
\end{center}
 
\noindent
where on the left-hand side is a single non-terminal and on the right a string consisting
of both terminals and non-terminals including the $\epsilon$-symbol for indicating the
empty string. We use the convention  to separate components on
the right hand-side by using the $\cdot$ symbol, as in the grammar for well-parenthesised  expressions.
We also use the convention to use $|$ as a shorthand notation for several rules. For example

\begin{center}
$NT \;\;\rightarrow\;\; \textit{rhs}_1 \;|\; \textit{rhs}_2$
\end{center}

\noindent
means that the non-terminal $NT$ can be replaced by either $\textit{rhs}_1$ or $\textit{rhs}_2$.
If there are more than one non-terminal on the left-hand side of the rules, then we need to indicate
what is the \emph{starting} symbol of the grammar. For example the grammar for arithmetic expressions
can be given as follows

\begin{center}
\begin{tabular}{lcl}
$E$ & $\rightarrow$ &  $N$ \\
$E$ & $\rightarrow$ &  $E \cdot + \cdot E$ \\
$E$ & $\rightarrow$ &  $E \cdot - \cdot E$ \\
$E$ & $\rightarrow$ &  $E \cdot * \cdot E$ \\
$E$ & $\rightarrow$ &  $( \cdot E \cdot )$\\
$N$ & $\rightarrow$ & $\epsilon \;|\; 0 \cdot N \;|\; 1 \cdot N \;|\: \ldots \;|\; 9 \cdot N$ 
\end{tabular}
\end{center}

\noindent
where $E$ is the starting symbol. A \emph{derivation} for a grammar
starts with the staring symbol of the grammar and in each step replaces one
non-terminal by a right-hand side of a rule.


\end{document}

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