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\begin{document}

\fnote{\copyright{} Christian Urban, King's College London, 2014, 2015, 2016}

\section*{Handout 1}

This module is about text processing, be it for web-crawlers,

compilers, dictionaries, DNA-data and so on. When looking for

a particular string, like $abc$ in a large text we can use the

Knuth-Morris-Pratt algorithm, which is currently the most

efficient general string search algorithm. But often we do

\emph{not} just look for a particular string, but for string

patterns. For example in program code we need to identify what

are the keywords, what are the identifiers etc. A pattern for

identifiers could be stated as: they start with a letter,

followed by zero or more letters, numbers and underscores.

Also often we face the problem that we are given a string (for

example some user input) and want to know whether it matches a

particular pattern---be it an email address, for example. In

this way we can exclude user input that would otherwise have

nasty effects on our program (crashing it or making it go into

an infinite loop, if not worse).\smallskip

\defn{Regular expressions} help with conveniently specifying

such patterns. The idea behind regular expressions is that

they are a simple method for describing languages (or sets of

strings)\ldots at least languages we are interested in in

computer science. For example there is no convenient regular

expression for describing the English language short of

enumerating all English words. But they seem useful for

describing for example simple email addresses.\footnote{See

``8 Regular Expressions You Should Know''

\url{http://goo.gl/5LoVX7}} Consider the following regular

expression

\begin{equation}\label{email}

\texttt{[a-z0-9\_.-]+} \;\;\texttt{@}\;\; \texttt{[a-z0-9.-]+} \;\;\texttt{.}\;\; \texttt{[a-z.]\{2,6\}}

\end{equation}

\noindent where the first part, the user name, matches one or more lowercase

letters (\pcode{a-z}), digits (\pcode{0-9}), underscores, dots

and hyphens. The \pcode{+} at the end of the brackets ensures

the ``one or more''. Then comes the \pcode{@}-sign, followed

by the domain name which must be one or more lowercase

letters, digits, underscores, dots or hyphens. Note there

cannot be an underscore in the domain name. Finally there must

be a dot followed by the toplevel domain. This toplevel domain

must be 2 to 6 lowercase letters including the dot. Example

strings which follow this pattern are:

\begin{lstlisting}[language={},numbers=none,keywordstyle=\color{black}]

niceandsimple@example.org

very.common@example.co.uk

a.little.lengthy.but.fine@dept.example.ac.uk

other.email-with-dash@example.edu

\end{lstlisting}

\noindent

But for example the following two do not

\begin{lstlisting}[language={},numbers=none,keywordstyle=\color{black}]

user@localserver

disposable.style.email.with+symbol@example.com

\end{lstlisting}

\noindent according to the regular expression we specified in

\eqref{email}. Whether this is intended or not is a different

question (the second email above is actually an acceptable

email address acording to the RFC 5322 standard for email

addresses).

As mentioned above, identifiers, or variables, in program code

are often required to satisfy the constraints that they start

with a letter and then can be followed by zero or more letters

or numbers and also can include underscores, but not as the

first character. Such identifiers can be recognised with the

regular expression

\begin{center}

\pcode{[a-zA-Z] [a-zA-Z0-9_]*}

\end{center}

\noindent Possible identifiers that match this regular expression 

are \pcode{x}, \pcode{foo}, \pcode{foo_bar_1}, \pcode{A_very_42_long_object_name},

but not \pcode{_i} and also not \pcode{4you}.

Many programming languages offer libraries that can be used to

validate such strings against regular expressions. Also there

are some common, and I am sure very familiar, ways of how to

construct regular expressions. For example in Scala we have

a library implementing the following regular expressions: 

\begin{center}

\begin{tabular}{lp{9cm}}

\pcode{re*} & matches 0 or more occurrences of preceding 

expression\\

\pcode{re+} & matches 1 or more occurrences of preceding

expression\\

\pcode{re?} &	 matches 0 or 1 occurrence of preceding 

expression\\

\pcode{re\{n\}}	& matches exactly \pcode{n} number of 

occurrences of preceding  expression\\

\pcode{re\{n,m\}} & matches at least \pcode{n} and at most {\tt m}

occurences of the preceding expression\\

\pcode{[...]} & matches any single character inside the 

brackets\\

\pcode{[^...]} & matches any single character not inside the 

brackets\\

\pcode{...-...} & character ranges\\

\pcode{\\d} & matches digits; equivalent to \pcode{[0-9]}\\

\pcode{.} & matches every character except newline\\

\pcode{(re)}	& groups regular expressions and remembers 

matched text

\end{tabular}

\end{center}

\noindent With this table you can figure out the purpose of

the regular expressions in the web-crawlers shown Figures

\ref{crawler1}, \ref{crawler2} and

\ref{crawler3}. Note, however, the regular expression for

http-addresses in web-pages in Figure~\ref{crawler1}, Line 15,

is intended to be

\[

\pcode{"https?://[^"]*"}

\]

\noindent It specifies that web-addresses need to start with a

double quote, then comes \texttt{http} followed by an optional

\texttt{s} and so on until the closing double quote comes.

Usually we would have to escape the double quotes in order to

make sure we interpret the double quote as character, not as

double quote for a string. But Scala's trick with triple

quotes allows us to omit this kind of escaping. As a result we

can just write:

\[

\pcode{""""https?://[^"]*"""".r}

\]

\noindent Note also that the convention in Scala is that

\texttt{.r} converts a string into a regular expression. I

leave it to you to ponder whether this regular expression

really captures all possible web-addresses.

\subsection*{Why Study Regular Expressions?}

Regular expressions were introduced by Kleene in the 1950ies

and they have been object of intense study since then. They

are nowadays pretty much ubiquitous in computer science. There

are many libraries implementing regular expressions. I am sure

you have come across them before (remember PRA?). Why on earth

then is there any interest in studying them again in depth in

this module? Well, one answer is in the following two graphs about

regular expression matching in Python, Ruby and Java.

\begin{center}

\begin{tabular}{@{\hspace{-1mm}}c@{\hspace{-1mm}}c@{}}

\begin{tikzpicture}

\begin{axis}[

    title={Graph: $\texttt{a?\{n\}\,a{\{n\}}}$ and strings 

           $\underbrace{\texttt{a}\ldots \texttt{a}}_{n}$},

    xlabel={$n$},

    x label style={at={(1.05,0.0)}},

    ylabel={time in secs},

    enlargelimits=false,

    xtick={0,5,...,30},

    xmax=33,

    ymax=35,

    ytick={0,5,...,30},

    scaled ticks=false,

    axis lines=left,

    width=5.5cm,

    height=4.5cm, 

    legend entries={Python,Ruby},  

    legend pos=north west,

    legend cell align=left]

\addplot[blue,mark=*, mark options={fill=white}] 

  table {re-python.data};

\addplot[brown,mark=triangle*, mark options={fill=white}] 

  table {re-ruby.data};  

\end{axis}

\end{tikzpicture}

&

\begin{tikzpicture}

\begin{axis}[

    title={Graph: $\texttt{(a*)*\,b}$ and strings 

           $\underbrace{\texttt{a}\ldots \texttt{a}}_{n}$},

    xlabel={$n$},

    x label style={at={(1.05,0.0)}},

    ylabel={time in secs},

    enlargelimits=false,

    xtick={0,5,...,30},

    xmax=33,

    ymax=35,

    ytick={0,5,...,30},

    scaled ticks=false,

    axis lines=left,

    width=5.5cm,

    height=4.5cm, 

    legend entries={Java},  

    legend pos=north west,

    legend cell align=left]

\addplot[cyan,mark=*, mark options={fill=white}] table {re-java.data};

\end{axis}

\end{tikzpicture}

\end{tabular}

\end{center}

\noindent This first graph shows that Python needs approximately 29

seconds for finding out whether a string of 28 \texttt{a}s

matches the regular expression \texttt{a?\{28\}\,a\{28\}}.

Ruby is even slightly worse.\footnote{In this example Ruby

uses the slightly different regular expression

\texttt{a?a?a?...a?a?aaa...aa}, where the \texttt{a?} and

\texttt{a} each occur $n$ times. More such test cases can be

found at \url{http://www.computerbytesman.com/redos/}.} Simlarly, Java needs approximately

30 seconds to find out that the regular expression $\texttt{(a*)*\,b}$

does not match strings of 28 \texttt{a}s.

Admittedly, these regular expressions are carefully chosen to

exhibit this exponential behaviour, but similar ones occur

more often than one wants in ``real life''. For example, on 20

July 2016 a similar regular expression brought the webpage

\href{http://stackexchange.com}{Stack Exchange} to its knees:

\begin{center}

\url{http://stackstatus.net/post/147710624694/outage-postmortem-july-20-2016}

\end{center}

\noindent

\begin{center}

\url{http://davidvgalbraith.com/how-i-fixed-atom/}

\end{center}

\noindent

Such troublesome regular expressions are sometimes called \emph{evil

  regular expressions} because they have the potential to make regular

expression matching engines to topple over, like in Python, Ruby and

Java. This ``toppling over'' is also sometimes called \emph{catastrophic

  backtracking}.  The problem with evil regular expressions is that

they can have some serious consequences, for example, if you use them

in your web-application. The reason is that hackers can look for these

instances where the matching engine behaves badly and then mount a

nice DoS-attack against your application. These attacks are already

have their own name: \emph{Regular Expression Denial of Service

  Attacks (ReDoS)}.

It will be instructive to look behind the ``scenes'' to find

out why Python and Ruby (and others) behave so badly when

matching strings with evil regular expressions. But we will also look

at a relatively simple algorithm that solves this problem much

better than Python and Ruby do\ldots actually it will be two

versions of the algorithm: the first one will be able to

process strings of approximately 1,000 \texttt{a}s in 30

seconds, while the second version will even be able to process

up to 12,000 in less than 10(!) seconds, see the graph below:

\begin{center}

\begin{tikzpicture}

  \begin{axis}[

    title={Graph: $\texttt{a?\{n\}\,a{\{n\}}}$ and strings 

           $\underbrace{\texttt{a}\ldots \texttt{a}}_{n}$},

    xlabel={$n$},

    x label style={at={(1.05,0.0)}},

    ylabel={time in secs},

    enlargelimits=false,

    xtick={0,3000,...,12000},

    xmax=13000,

    ymax=12,

    ytick={0,5,10},

    scaled ticks=false,

    axis lines=left,

    width=7cm,

    height=4.5cm,

    legend entries={Our Algorithm V1, Our Algorithm V2},

    legend pos=outer north east]

\addplot[green,mark=square*,mark options={fill=white}] table {re2.data};

\addplot[black,mark=square*,mark options={fill=white}] table {re3.data};

\end{axis}

\end{tikzpicture}

\end{center}

\noindent And in the case of the regular expression $\texttt{(a*)*\,b}$

and strings of \texttt{a}s we will beat Java by factor of

approximately 1,000,000 (note the scale on the $x$-axis). 

\begin{center}

\begin{tikzpicture}

  \begin{axis}[

    title={Graph: $\texttt{(a*)*\,b}$ and strings 

           $\underbrace{\texttt{a}\ldots \texttt{a}}_{n}$},

    xlabel={$n$},

    x label style={at={(1.05,0.0)}},

    ylabel={time in secs},

    enlargelimits=false,

    ymax=12,

    ytick={0,5,10},

    axis lines=left,

    width=7cm,

    height=4.5cm,

    legend entries={Our Algorithm V2},

    legend pos=outer north east]

\addplot[black,mark=square*,mark options={fill=white}] table {re3a.data};

\end{axis}

\end{tikzpicture}

\end{center}

\subsection*{Basic Regular Expressions}

The regular expressions shown earlier for Scala, we

will call \emph{extended regular expressions}. The ones we

will mainly study in this module are \emph{basic regular

expressions}, which by convention we will just call

\emph{regular expressions}, if it is clear what we mean. The

attraction of (basic) regular expressions is that many

features of the extended ones are just syntactic sugar.

(Basic) regular expressions are defined by the following

grammar:

\begin{center}

\begin{tabular}{r@{\hspace{1mm}}r@{\hspace{1mm}}l@{\hspace{13mm}}l}

  $r$ & $::=$ &    $\ZERO$          & null language\\

        & $\mid$ & $\ONE$           & empty string / \texttt{""} / []\\

        & $\mid$ & $c$                  & single character\\

        & $\mid$ & $r_1 + r_2$          & alternative / choice\\

        & $\mid$ & $r_1 \cdot r_2$      & sequence\\

        & $\mid$ & $r^*$                & star (zero or more)\\

  \end{tabular}

\end{center}

\noindent Because we overload our notation, there are some

subtleties you should be aware of. When regular expressions

are referred to, then $\ZERO$ (in bold font) does not stand for

the number zero: rather it is a particular pattern that does

not match any string. Similarly, in the context of regular

expressions, $\ONE$ does not stand for the number one but for

a regular expression that matches the empty string. The letter

$c$ stands for any character from the alphabet at hand. Again

in the context of regular expressions, it is a particular

pattern that can match the specified character. You should

also be careful with our overloading of the star: assuming you

have read the handout about our basic mathematical notation,

you will see that in the context of languages (sets of

strings) the star stands for an operation on languages. Here

$r^*$ stands for a regular expression, which is different from

the operation on sets is defined as

\[

A\star\dn \bigcup_{0\le n} A^n

\]

We will use parentheses to disambiguate regular expressions.

Parentheses are not really part of a regular expression, and

indeed we do not need them in our code because there the tree

structure of regular expressions is always clear. But for

writing them down in a more mathematical fashion, parentheses

will be helpful. For example we will write $(r_1 + r_2)^*$,

which is different from, say $r_1 + (r_2)^*$. The former means

roughly zero or more times $r_1$ or $r_2$, while the latter

means $r_1$ or zero or more times $r_2$. This will turn out to

be two different patterns, which match in general different

strings. We should also write $(r_1 + r_2) + r_3$, which is

different from the regular expression $r_1 + (r_2 + r_3)$, but

in case of $+$ and $\cdot$ we actually do not care about the

order and just write $r_1 + r_2 + r_3$, or $r_1 \cdot r_2

\cdot r_3$, respectively. The reasons for this will become

clear shortly. 

In the literature you will often find that the choice $r_1 +

r_2$ is written as $r_1\mid{}r_2$ or $r_1\mid\mid{}r_2$. Also,

often our $\ZERO$ and $\ONE$ are written $\varnothing$ and

$\epsilon$, respectively. Following the convention in the

literature, we will often omit the $\cdot$ all together. This

is to make some concrete regular expressions more readable.

For example the regular expression for email addresses shown

in \eqref{email} would look like

\[

\texttt{[...]+} \;\cdot\;  \texttt{@} \;\cdot\; 

\texttt{[...]+} \;\cdot\; \texttt{.} \;\cdot\; 

\texttt{[...]\{2,6\}}

\]

\noindent

which is much less readable than \eqref{email}. Similarly for

the regular expression that matches the string $hello$ we 

should write

\[

h \cdot e \cdot l \cdot l \cdot o

\]

\noindent

but often just write {\it hello}.

If you prefer to think in terms of the implementation

of regular expressions in Scala, the constructors and

classes relate as follows\footnote{More about Scala is 

in the handout about \emph{A Crash-Course on Scala}.}

\begin{center}

\begin{tabular}{rcl}

$\ZERO$       & $\mapsto$ & \texttt{ZERO}\\

$\ONE$        & $\mapsto$ & \texttt{ONE}\\

$c$           & $\mapsto$ & \texttt{CHAR(c)}\\

$r_1 + r_2$   & $\mapsto$ & \texttt{ALT(r1, r2)}\\

$r_1 \cdot r_2$ & $\mapsto$ & \texttt{SEQ(r1, r2)}\\

$r^*$         & $\mapsto$ & \texttt{STAR(r)}

\end{tabular}

\end{center}

A source of confusion might arise from the fact that we

use the term \emph{basic regular expression} for the regular

expressions used in ``theory'' and defined above, and

\emph{extended regular expression} for the ones used in

``practice'', for example in Scala. If runtime is not an

issue, then the latter can be seen as syntactic sugar of

the former. For example we could replace

\begin{center}

\begin{tabular}{rcl}

$r+$ & $\mapsto$ & $r\cdot r^*$\\

$r?$ & $\mapsto$ & $\ONE + r$\\

$\backslash d$ & $\mapsto$ & $0 + 1 + 2 + \ldots + 9$\\

$[\text{\it a - z}]$ & $\mapsto$ & $a + b + \ldots + z$\\

\end{tabular}

\end{center}

\subsection*{The Meaning of Regular Expressions}

So far we have only considered informally what the

\emph{meaning} of a regular expression is. This is not good

enough for specifications of what algorithms are supposed to

do or which problems they are supposed to solve.

To define the meaning of a regular expression we will

associate with every regular expression a language, or set of

strings. This language contains all the strings the regular

expression is supposed to match. To understand what is going

on here it is crucial that you have read the handout

about basic mathematical notations.

The \defn{meaning of a regular expression} can be defined

by a recursive function called $L$ (for language), which

is defined as follows