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

\section*{Handout 4 (Sulzmann \& Lu Algorithm)}

So far our algorithm based on derivatives was only able to say

yes or no depending on whether a string was matched by a regular

expression or not. Often a more interesting question is to

find out \emph{how} a regular expression matched a string?

Answering this question will also help us with the problem we

are after, namely tokenising an input string. 

The algorithm we will be looking at in this lecture was designed by Sulzmann

\& Lu in a rather recent research paper (from 2014). A link to it is

provided on KEATS, in case you are interested.\footnote{In my

humble opinion this is an interesting instance of the research

literature: it contains a very neat idea, but its presentation

is rather sloppy. In earlier versions of this paper, a King's

undergraduate student and I found several rather annoying typos in the

examples and definitions.} My PhD student Fahad Ausaf and I even more recently 

wrote a paper where we build on their result: we provided a 

mathematical proof that their algorithm is really correct---the proof 

Sulzmann \& Lu had originally given contained major flaws. Such correctness

proofs are important: Kuklewicz maintains a unit-test library

for the kind of algorithma we are interested in here and he showed 

that many implementations in the ``wild'' are buggy, that is not

satisfy his unit tests:

\begin{center}

\url{http://www.haskell.org/haskellwiki/Regex_Posix}

\end{center}

In order to give an answer for

\emph{how} a regular expression matches a string, Sulzmann and

Lu use \emph{values}. A value will be the output of the

algorithm whenever the regular expression matches the string.

If the string does not match the string, an error will be

raised. 

The definition for values is given below. They are shown 

together with the regular expressions $r$ to which

they correspond:

\begin{center}

\begin{tabular}{cc}

\begin{tabular}{@{}rrl@{}}

\multicolumn{3}{c}{regular expressions}\medskip\\

  $r$ & $::=$  & $\ZERO$\\

      & $\mid$ & $\ONE$   \\

      & $\mid$ & $c$          \\

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

      & $\mid$ & $r_1 + r_2$   \\

  \\

      & $\mid$ & $r^*$         \\

\end{tabular}

&

\begin{tabular}{@{\hspace{0mm}}rrl@{}}

\multicolumn{3}{c}{values}\medskip\\

   $v$ & $::=$  & \\

      &        & $Empty$   \\

      & $\mid$ & $Char(c)$          \\

      & $\mid$ & $Seq(v_1,v_2)$\\

      & $\mid$ & $\Left(v)$   \\

      & $\mid$ & $Right(v)$  \\

      & $\mid$ & $Stars\,[v_1,\ldots\,v_n]$ \\

\end{tabular}

\end{tabular}

\end{center}

\noindent There is no value for the

$\ZERO$ regular expression, since it does not match any

string. Otherwise there is exactly one value corresponding to

each regular expression with the exception of $r_1 + r_2$

where there are two values, namely $\Left(v)$ and $Right(v)$

corresponding to the two alternatives. Note that $r^*$ is

associated with a list of values, one for each copy of $r$

that was needed to match the string. This means we might also

return the empty list $Stars []$, if no copy was needed in case

of $r^*$. For sequence, there is exactly one value, composed 

of two component values ($v_1$ and $v_2$).

My implementation of regular expressions and values in Scala is

shown below. I have in my implementation the convention that

regular expressions are written entirely with upper-case

letters, while values just start with a single upper-case

character and the rest are lower-case letters.

{../progs/app01.scala}}

{../progs/app02.scala}}

Graphically the algorithm by Sulzmann \& Lu can be illustrated

by the picture in Figure~\ref{Sulz} where the path from the

left to the right involving $\textit{der}/\textit{nullable}$ is the first phase

of the algorithm and $\textit{mkeps}/\textit{inj}$, the path from right to left,

the second phase. This picture shows the steps required when a

regular expression, say $r_1$, matches the string $abc$. We

first build the three derivatives (according to $a$, $b$ and

$c$). We then use $\textit{nullable}$ to find out whether the resulting

regular expression can match the empty string. If yes, we call

the function $\textit{mkeps}$. The $\textit{mkeps}$ function calculates a value for how a regular

expression has matched the empty string. Its definition

is as follows:

\begin{figure}[t]

\begin{center}

\begin{tikzpicture}[scale=2,node distance=1.2cm,

                    every node/.style={minimum size=7mm}]

\node (r1)  {$r_1$};

\node (r2) [right=of r1]{$r_2$};

\draw[->,line width=1mm](r1)--(r2) node[above,midway] {$\textit{der}\,a$};

\node (r3) [right=of r2]{$r_3$};

\draw[->,line width=1mm](r2)--(r3) node[above,midway] {$\textit{der}\,b$};

\node (r4) [right=of r3]{$r_4$};

\draw[->,line width=1mm](r3)--(r4) node[above,midway] {$\textit{der}\,c$};

\draw (r4) node[anchor=west] {\;\raisebox{3mm}{$\textit{nullable}$}};

\node (v4) [below=of r4]{$v_4$};

\draw[->,line width=1mm](r4) -- (v4);

\node (v3) [left=of v4] {$v_3$};

\draw[->,line width=1mm](v4)--(v3) node[below,midway] {$\textit{inj}\,c$};

\node (v2) [left=of v3]{$v_2$};

\draw[->,line width=1mm](v3)--(v2) node[below,midway] {$\textit{inj}\,b$};

\node (v1) [left=of v2] {$v_1$};

\draw[->,line width=1mm](v2)--(v1) node[below,midway] {$\textit{inj}\,a$};

\draw (r4) node[anchor=north west] {\;\raisebox{-8mm}{$\textit{mkeps}$}};

\end{tikzpicture}

\end{center}

\caption{The two phases of the algorithm by Sulzmann \& Lu.

\label{Sulz}}

\end{figure}

\begin{center}

\begin{tabular}{lcl}

  $\textit{mkeps}(\ONE)$       & $\dn$ & $Empty$\\

  $\textit{mkeps}(r_1 + r_2)$      & $\dn$ & if $\textit{nullable}(r_1)$  \\

                          &       & then $\Left(\textit{mkeps}(r_1))$\\

                          &       & else $Right(\textit{mkeps}(r_2))$\\

  $\textit{mkeps}(r_1 \cdot r_2)$  & $\dn$ & $Seq(\textit{mkeps}(r_1),\textit{mkeps}(r_2))$\\

  $\textit{mkeps}(r^*)$            & $\dn$ & $Stars\,[]$  \\

\end{tabular}

\end{center}

\noindent There is are no cases for $\ZERO$ and $c$, since

these regular expression cannot match the empty string. Note

also that in case of alternatives we give preference to the

regular expression on the left-hand side. This will become

important later on.

The second phase of the algorithm is organised so that it will

calculate a value for how the derivative regular expression

has matched a string. For this we need a function that

reverses this ``chopping off'' for values which we did in the

first phase for derivatives. The corresponding function is

called $\textit{inj}$ (short for injection). This function takes three

arguments: the first one is a regular expression for which we

want to calculate the value, the second is the character we

want to inject and the third argument is the value where we

will inject the character into. The result of this function is a

new value. The definition of $\textit{inj}$ is as follows: 

\begin{center}

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

  $\textit{inj}\,(c)\,c\,Empty$            & $\dn$ & $Char\,c$\\

  $\textit{inj}\,(r_1 + r_2)\,c\,\Left(v)$ & $\dn$ & $\Left(\textit{inj}\,r_1\,c\,v)$\\

  $\textit{inj}\,(r_1 + r_2)\,c\,Right(v)$ & $\dn$ & $Right(\textit{inj}\,r_2\,c\,v)$\\

  $\textit{inj}\,(r_1 \cdot r_2)\,c\,Seq(v_1,v_2)$ & $\dn$  & $Seq(\textit{inj}\,r_1\,c\,v_1,v_2)$\\

  $\textit{inj}\,(r_1 \cdot r_2)\,c\,\Left(Seq(v_1,v_2))$ & $\dn$  & $Seq(\textit{inj}\,r_1\,c\,v_1,v_2)$\\

  $\textit{inj}\,(r_1 \cdot r_2)\,c\,Right(v)$ & $\dn$  & $Seq(\textit{mkeps}(r_1),\textit{inj}\,r_2\,c\,v)$\\

  $\textit{inj}\,(r^*)\,c\,Seq(v,Stars\,vs)$         & $\dn$  & $Stars(\textit{inj}\,r\,c\,v\,::\,vs)$\\

\end{tabular}

\end{center}

\noindent This definition is by recursion on the regular

expression and by analysing the shape of the values. Therefore

there are three cases for sequence regular

expressions (for all possible shapes of the value we can encounter). The last

clause for the star regular expression returns a list where

the first element is $\textit{inj}\,r\,c\,v$ and the other elements are

$vs$. That means $\_\,::\,\_$ should be read as list cons.

To understand what is going on, it might be best to do some

example calculations and compare them with Figure~\ref{Sulz}.

For this note that we have not yet dealt with the need of

simplifying regular expressions (this will be a topic on its

own later). Suppose the regular expression is $a \cdot (b

\cdot c)$ and the input string is $abc$. The derivatives from

the first phase are as follows:

\begin{center}

\begin{tabular}{ll}

$r_1$: & $a \cdot (b \cdot c)$\\

$r_2$: & $\ONE \cdot (b \cdot c)$\\

$r_3$: & $(\ZERO \cdot (b \cdot c)) + (\ONE \cdot c)$\\

$r_4$: & $(\ZERO \cdot (b \cdot c)) + ((\ZERO \cdot c) + \ONE)$\\

\end{tabular}

\end{center}

\noindent According to the simple algorithm, we would test

whether $r_4$ is nullable, which in this case it indeed is.

This means we can use the function $\textit{mkeps}$ to calculate a

value for how $r_4$ was able to match the empty string.

Remember that this function gives preference for alternatives

on the left-hand side. However there is only $\ONE$ on the

very right-hand side of $r_4$ (underlined)

\begin{center}

$r_4$:\;$(\ZERO \cdot (b \cdot c)) + 

         ((\ZERO \cdot c) + \underline{\ONE})$\\

\end{center}

\noindent

that matches the empty string.

Therefore $\textit{mkeps}$ returns the value

\begin{center}

$v_4:\;Right(Right(Empty))$

\end{center}

\noindent If there had been a $\ONE$ on the left, then

$\textit{mkeps}$ would have returned something of the form

$\Left(\ldots)$. The point is that from this value we can

directly read off which part of $r_4$ matched the empty

string: take the right-alternative first, and then the

right-alternative again. 

Next we have to ``inject'' the last character, that is $c$ in

the running example, into this value $v_4$ in order to

calculate how $r_3$ could have matched the string $c$.

According to the definition of $\textit{inj}$ we obtain

\begin{center}

$v_3:\;Right(Seq(Empty, Char(c)))$

\end{center}

\noindent This is the correct result, because $r_3$ needs

to use the right-hand alternative, and then $\ONE$ needs

to match the empty string and $c$ needs to match $c$.

Next we need to inject back the letter $b$ into $v_3$. This

gives

\begin{center}

$v_2:\;Seq(Empty, Seq(Char(b), Char(c)))$

\end{center}

\noindent which is again the correct result for how $r_2$

matched the string $bc$. Finally we need to inject back the

letter $a$ into $v_2$ giving the final result

\begin{center}

$v_1:\;Seq(Char(a), Seq(Char(b), Char(c)))$

\end{center}

\noindent This now corresponds to how the regular

expression $a \cdot (b \cdot c)$ matched the string $abc$.

There are a few auxiliary functions that are of interest

when analysing this algorithm. One is called \emph{flatten},

written $|\_|$, which extracts the string ``underlying'' a 

value. It is defined recursively as

\begin{center}

\begin{tabular}{lcl}

  $|Empty|$     & $\dn$ & $[]$\\

  $|Char(c)|$   & $\dn$ & $[c]$\\

  $|\Left(v)|$   & $\dn$ & $|v|$\\

  $|Right(v)|$  & $\dn$ & $|v|$\\

  $|Seq(v_1,v_2)|$& $\dn$ & $|v_1| \,@\, |v_2|$\\

  $|Stars\,[v_1,\ldots ,v_n]|$ & $\dn$ & $|v_1| \,@\ldots @\, |v_n|$\\

\end{tabular}

\end{center}

\noindent Using flatten we can see what are the strings behind 

the values calculated by $\textit{mkeps}$ and $\textit{inj}$. In our running 

example:

\begin{center}

\begin{tabular}{ll}

$|v_4|$: & $[]$\\

$|v_3|$: & $c$\\

$|v_2|$: & $bc$\\

$|v_1|$: & $abc$

\end{tabular}

\end{center}

\noindent This indicates that $\textit{inj}$ indeed is injecting, or

adding, back a character into the value. 

There is a problem, however, with the described algorithm

so far: it is very slow. We need to include the simplification 

from Lecture 2. This is what we shall do next.

\subsubsection*{Simplification}

Generally the matching algorithms based on derivatives do

poorly unless the regular expressions are simplified after

each derivative step. But this is a bit more involved in the

algorithm of Sulzmann \& Lu. So what follows might require you

to read several times before it makes sense and also might

require that you do some example calculations yourself. As a

first example consider the last derivation step in our earlier

example:

\begin{equation}\label{regexfour}

r_4 = \textit{der}\,c\,r_3 = 

(\ZERO \cdot (b \cdot c)) + ((\ZERO \cdot c) + \ONE)

\end{equation}

\noindent Simplifying this regular expression would just give

us $\ONE$. Running $\textit{mkeps}$ with this $\ONE$ as

input, however, would give us with the value $Empty$ instead of

\[Right(Right(Empty))\]

\noindent

that was obtained  without the simplification. The problem  is we need

to  recreate this  more  complicated value,  rather  than just  return

$Empty$. This will require what I call \emph{rectification functions}.

They  need  to  be  calculated  whenever  a  regular  expression  gets

simplified.

Rectification functions take a value as argument and return a

(rectified) value. In the example above the argument would be $Empty$

and the output $Right(Right(Empty))$.  Before we define these

rectification functions in general, let us first take a look again at

our simplification rules:

\begin{center}

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

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

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

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

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

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

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

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

\end{tabular}

\end{center}

\noindent Applying them to $r_4$ in \eqref{regexfour} will require several nested

simplifications in order end up with just $\ONE$. However,

it is possible to apply them in a depth-first, or inside-out,

manner in order to calculate this simplified regular

expression.

The rectification we can implement by letting simp return not

just a (simplified) regular expression, but also a

rectification function. Let us consider the alternative case,

$r_1 + r_2$, first. By going depth-first, we first simplify

the component regular expressions $r_1$ and $r_2.$ This will

return simplified versions, say $r_{1s}$ and $r_{2s}$, of the

component regular expressions (if they can be simplified) but

also two rectification functions $f_{1s}$ and $f_{2s}$. We

need to assemble them in order to obtain a rectified value for

$r_1 + r_2$. In case $r_{1s}$ simplified to $\ZERO$, we

continue the derivative calculation with $r_{2s}$. The

Sulzmann \& Lu algorithm would return a corresponding value,

say $v_{2s}$. But now this value needs to be ``rectified'' to

the value 

\begin{center}

$Right(v_{2s})$

\end{center}

\noindent The reason is that we look for the value that tells

us how $r_1 + r_2$ could have matched the string, not just

$r_2$ or $r_{2s}$. Unfortunately, this is still not the right

value in general because there might be some simplifications

that happened inside $r_2$ and for which the simplification

function retuned also a rectification function $f_{2s}$. So in

fact we need to apply this one too which gives

\begin{center}

$Right(f_{2s}(v_{2s}))$

\end{center}

\noindent This is now the correct, or rectified, value. Since

the simplification will be done in the first phase of the

algorithm, but the rectification needs to be done to the

values in the second phase, it is advantageous to calculate

the rectification as a function, remember this function and

then apply the value to this function during the second phase.

So if we want to implement the rectification as function, we 

would need to return

\begin{center}

$\lambda v.\,Right(f_{2s}(v))$

\end{center}

\noindent which is the lambda-calculus notation for

a function that expects a value $v$ and returns everything

after the dot where $v$ is replaced by whatever value is 

given.

Let us package this idea with rectification functions

into a single function (still only considering the alternative

case):

\begin{center}

\begin{tabular}{ll}

$_1$ & $simp(r)$:\\

$_2$ & \quad case $r = r_1 + r_2$\\

$_3$ & \qquad let $(r_{1s}, f_{1s}) = simp(r_1)$\\

$_4$ & \qquad \phantom{let} $(r_{2s}, f_{2s}) = simp(r_2)$\smallskip\\

$_5$ & \qquad case $r_{1s} = \ZERO$: 

       return $(r_{2s}, \lambda v. \,Right(f_{2s}(v)))$\\

$_6$ & \qquad case $r_{2s} = \ZERO$: 

       return $(r_{1s}, \lambda v. \,\Left(f_{1s}(v)))$\\

$_7$ & \qquad case $r_{1s} = r_{2s}$:

       return $(r_{1s}, \lambda v. \,\Left(f_{1s}(v)))$\\

$_8$ & \qquad otherwise: 

       return $(r_{1s} + r_{2s}, f_{alt}(f_{1s}, f_{2s}))$\\

\end{tabular}

\end{center}

\noindent We first recursively call the simplification with

$r_1$ and $r_2$ (Lines 3 and 4). This gives simplified regular expressions,

$r_{1s}$ and $r_{2s}$, as well as two rectification functions

$f_{1s}$ and $f_{2s}$. We next need to test whether the

simplified regular expressions are $\ZERO$ so as to make

further simplifications. In case $r_{1s}$ is $\ZERO$ (Line 5),

then we can return $r_{2s}$ (the other alternative). However

for this we need to build a corresponding rectification 

function, which as said above is

\begin{center}

$\lambda v.\,Right(f_{2s}(v))$

\end{center}

\noindent The case where $r_{2s} = \ZERO$ is similar:

We return $r_{1s}$ and rectify with $\Left(\_)$ and the

other calculated rectification function $f_{1s}$. This gives

\begin{center}

$\lambda v.\,\Left(f_{1s}(v))$

\end{center}

\noindent The next case where $r_{1s} = r_{2s}$ can be treated

like the one where $r_{2s} = \ZERO$. We return $r_{1s}$

and rectify with $\Left(\_)$ and so on.

The otherwise-case is slightly more complicated. In this case

neither $r_{1s}$ nor $r_{2s}$ are $\ZERO$ and also

$r_{1s} \not= r_{2s}$, which means no further simplification

can be applied. Accordingly, we return $r_{1s} + r_{2s}$ as

the simplified regular expression. In principle we also do not

have to do any rectification, because no simplification was

done in this case. But this is actually not true: There might

have been simplifications inside $r_{1s}$ and $r_{2s}$. We

therefore need to take into account the calculated

rectification functions $f_{1s}$ and $f_{2s}$. We can do this

by defining a rectification function $f_{alt}$ which takes two

rectification functions as arguments and applies them

according to whether the value is of the form $\Left(\_)$ or

$Right(\_)$:

\begin{center}

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

$f_{alt}(f_1, f_2) \dn$\\

\qquad $\lambda v.\,$ case $v = \Left(v')$: 

      & return $\Left(f_1(v'))$\\

\qquad \phantom{$\lambda v.\,$} case $v = Right(v')$: 

      & return $Right(f_2(v'))$\\      

\end{tabular}

\end{center}

The other interesting case with simplification is the sequence

case. In this case the main simplification function is as

follows