--- a/etnms/etnms.tex Wed Feb 05 11:27:51 2020 +0000
+++ b/etnms/etnms.tex Wed Feb 05 11:54:51 2020 +0000
@@ -531,15 +531,15 @@
$(_{bs}\textit{CHAR}\;d)\,\backslash c$ & $\dn$ &
$\textit{if}\;c=d\; \;\textit{then}\;
_{bs}\textit{ONE}\;\textit{else}\;\textit{ZERO}$\\
- $(_{bs}\textit{ALTS}\;as)\,\backslash c$ & $\dn$ &
- $_{bs}\textit{ALTS}\;(as.map(\backslash c))$\\
+ $(_{bs}\textit{ALTS}\;\textit{as})\,\backslash c$ & $\dn$ &
+ $_{bs}\textit{ALTS}\;(\textit{as}.\textit{map}(\backslash c))$\\
$(_{bs}\textit{SEQ}\;a_1\,a_2)\,\backslash c$ & $\dn$ &
$\textit{if}\;\textit{bnullable}\,a_1$\\
& &$\textit{then}\;_{bs}\textit{ALTS}\,List((_{[]}\textit{SEQ}\,(a_1\,\backslash c)\,a_2),$\\
& &$\phantom{\textit{then}\;_{bs}\textit{ALTS}\,}(\textit{fuse}\,(\textit{bmkeps}\,a_1)\,(a_2\,\backslash c)))$\\
& &$\textit{else}\;_{bs}\textit{SEQ}\,(a_1\,\backslash c)\,a_2$\\
$(_{bs}\textit{STAR}\,a)\,\backslash c$ & $\dn$ &
- $_{bs}\textit{SEQ}\;(\textit{fuse}\, [\Z] (r\,\backslash c))\,
+ $_{bs}\textit{SEQ}\;(\textit{fuse}\, [0] \; r\,\backslash c )\,
(_{bs}\textit{STAR}\,[]\,r)$
\end{tabular}
\end{center}
@@ -553,36 +553,37 @@
$(_{bs}{\bf d})\,\backslash c$ & $\dn$ &
$\textit{if}\;c=d\; \;\textit{then}\;
_{bs}\ONE\;\textit{else}\;\ZERO$\\
- $(_{bs}\oplus \;as)\,\backslash c$ & $\dn$ &
- $_{bs}\oplus\;(as.map(\backslash c))$\\
+ $(_{bs}\oplus \;\textit{as})\,\backslash c$ & $\dn$ &
+ $_{bs}\oplus\;(\textit{as.map}(\backslash c))$\\
$(_{bs}\;a_1\cdot a_2)\,\backslash c$ & $\dn$ &
$\textit{if}\;\textit{bnullable}\,a_1$\\
& &$\textit{then}\;_{bs}\oplus\,[(_{[]}\,(a_1\,\backslash c)\cdot\,a_2),$\\
& &$\phantom{\textit{then},\;_{bs}\oplus\,}(\textit{fuse}\,(\textit{bmkeps}\,a_1)\,(a_2\,\backslash c))]$\\
& &$\textit{else}\;_{bs}\,(a_1\,\backslash c)\cdot a_2$\\
$(_{bs}a^*)\,\backslash c$ & $\dn$ &
- $_{bs}\;((\textit{fuse}\, [\Z] (r\,\backslash c))\cdot
+ $_{bs}\;((\textit{fuse}\, [0] (r\,\backslash c))\cdot
(_{[]}r^*))$
\end{tabular}
\end{center}
%\end{definition}
\noindent
-For instance, when we unfold $\textit{STAR} \; bs \; a$ into a sequence,
-we need to attach an additional bit $Z$ to the front of $r \backslash c$
-to indicate that there is one more star iteration. Also the $SEQ$ clause
+For instance, when we unfold $_{bs}a^*$ into a sequence,
+we need to attach an additional bit $0$ to the front of $r \backslash c$
+to indicate that there is one more star iteration. Also the sequence clause
is more subtle---when $a_1$ is $\textit{bnullable}$ (here
\textit{bnullable} is exactly the same as $\textit{nullable}$, except
that it is for annotated regular expressions, therefore we omit the
-definition). Assume that $bmkeps$ correctly extracts the bitcode for how
+definition). Assume that $\textit{bmkeps}$ correctly extracts the bitcode for how
$a_1$ matches the string prior to character $c$ (more on this later),
-then the right branch of $ALTS$, which is $fuse \; bmkeps \; a_1 (a_2
+then the right branch of alternative, which is $\textit{fuse} \; \bmkeps \; a_1 (a_2
\backslash c)$ will collapse the regular expression $a_1$(as it has
already been fully matched) and store the parsing information at the
head of the regular expression $a_2 \backslash c$ by fusing to it. The
-bitsequence $bs$, which was initially attached to the head of $SEQ$, has
-now been elevated to the top-level of $ALTS$, as this information will be
-needed whichever way the $SEQ$ is matched---no matter whether $c$ belongs
+bitsequence $\textit{bs}$, which was initially attached to the
+first element of the sequence $a_1 \cdot a_2$, has
+now been elevated to the top-level of $\oplus$, as this information will be
+needed whichever way the sequence is matched---no matter whether $c$ belongs
to $a_1$ or $ a_2$. After building these derivatives and maintaining all
the lexing information, we complete the lexing by collecting the
bitcodes using a generalised version of the $\textit{mkeps}$ function
@@ -1725,1298 +1726,14 @@
\noindent\rule[0.5ex]{\linewidth}{1pt}
-This PhD-project is about regular expression matching and
-lexing. Given the maturity of this topic, the reader might wonder:
-Surely, regular expressions must have already been studied to death?
-What could possibly be \emph{not} known in this area? And surely all
-implemented algorithms for regular expression matching are blindingly
-fast?
-
-Unfortunately these preconceptions are not supported by evidence: Take
-for example the regular expression $(a^*)^*\,b$ and ask whether
-strings of the form $aa..a$ match this regular
-expression. Obviously this is not the case---the expected $b$ in the last
-position is missing. One would expect that modern regular expression
-matching engines can find this out very quickly. Alas, if one tries
-this example in JavaScript, Python or Java 8 with strings like 28
-$a$'s, one discovers that this decision takes around 30 seconds and
-takes considerably longer when adding a few more $a$'s, as the graphs
-below show:
-
-\begin{center}
-\begin{tabular}{@{}c@{\hspace{0mm}}c@{\hspace{0mm}}c@{}}
-\begin{tikzpicture}
-\begin{axis}[
- xlabel={$n$},
- x label style={at={(1.05,-0.05)}},
- 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=5cm,
- height=4cm,
- legend entries={JavaScript},
- legend pos=north west,
- legend cell align=left]
-\addplot[red,mark=*, mark options={fill=white}] table {re-js.data};
-\end{axis}
-\end{tikzpicture}
- &
-\begin{tikzpicture}
-\begin{axis}[
- xlabel={$n$},
- x label style={at={(1.05,-0.05)}},
- %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=5cm,
- height=4cm,
- legend entries={Python},
- legend pos=north west,
- legend cell align=left]
-\addplot[blue,mark=*, mark options={fill=white}] table {re-python2.data};
-\end{axis}
-\end{tikzpicture}
- &
-\begin{tikzpicture}
-\begin{axis}[
- xlabel={$n$},
- x label style={at={(1.05,-0.05)}},
- %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=5cm,
- height=4cm,
- legend entries={Java 8},
- legend pos=north west,
- legend cell align=left]
-\addplot[cyan,mark=*, mark options={fill=white}] table {re-java.data};
-\end{axis}
-\end{tikzpicture}\\
-\multicolumn{3}{c}{Graphs: Runtime for matching $(a^*)^*\,b$ with strings
- of the form $\underbrace{aa..a}_{n}$.}
-\end{tabular}
-\end{center}
-
-\noindent These are clearly abysmal and possibly surprising results. One
-would expect these systems to do much better than that---after all,
-given a DFA and a string, deciding whether a string is matched by this
-DFA should be linear in terms of the size of the regular expression and
-the string?
-
-Admittedly, the regular expression $(a^*)^*\,b$ is carefully chosen to
-exhibit this super-linear behaviour. But unfortunately, such regular
-expressions are not just a few outliers. They are actually
-frequent enough to have a separate name created for
-them---\emph{evil regular expressions}. In empiric work, Davis et al
-report that they have found thousands of such evil regular expressions
-in the JavaScript and Python ecosystems \cite{Davis18}. Static analysis
-approach that is both sound and complete exists\cite{17Bir}, but the running
-time on certain examples in the RegExLib and Snort regular expressions
-libraries is unacceptable. Therefore the problem of efficiency still remains.
-
-This superlinear blowup in matching algorithms sometimes causes
-considerable grief in real life: for example on 20 July 2016 one evil
-regular expression brought the webpage
-\href{http://stackexchange.com}{Stack Exchange} to its
-knees.\footnote{\url{https://stackstatus.net/post/147710624694/outage-postmortem-july-20-2016}}
-In this instance, a regular expression intended to just trim white
-spaces from the beginning and the end of a line actually consumed
-massive amounts of CPU-resources---causing web servers to grind to a
-halt. This happened when a post with 20,000 white spaces was submitted,
-but importantly the white spaces were neither at the beginning nor at
-the end. As a result, the regular expression matching engine needed to
-backtrack over many choices. In this example, the time needed to process
-the string was $O(n^2)$ with respect to the string length. This
-quadratic overhead was enough for the homepage of Stack Exchange to
-respond so slowly that the load balancer assumed there must be some
-attack and therefore stopped the servers from responding to any
-requests. This made the whole site become unavailable. Another very
-recent example is a global outage of all Cloudflare servers on 2 July
-2019. A poorly written regular expression exhibited exponential
-behaviour and exhausted CPUs that serve HTTP traffic. Although the
-outage had several causes, at the heart was a regular expression that
-was used to monitor network
-traffic.\footnote{\url{https://blog.cloudflare.com/details-of-the-cloudflare-outage-on-july-2-2019/}}
-
-The underlying problem is that many ``real life'' regular expression
-matching engines do not use DFAs for matching. This is because they
-support regular expressions that are not covered by the classical
-automata theory, and in this more general setting there are quite a few
-research questions still unanswered and fast algorithms still need to be
-developed (for example how to treat efficiently bounded repetitions, negation and
-back-references).
-%question: dfa can have exponential states. isn't this the actual reason why they do not use dfas?
-%how do they avoid dfas exponential states if they use them for fast matching?
-
-There is also another under-researched problem to do with regular
-expressions and lexing, i.e.~the process of breaking up strings into
-sequences of tokens according to some regular expressions. In this
-setting one is not just interested in whether or not a regular
-expression matches a string, but also in \emph{how}. Consider for
-example a regular expression $r_{key}$ for recognising keywords such as
-\textit{if}, \textit{then} and so on; and a regular expression $r_{id}$
-for recognising identifiers (say, a single character followed by
-characters or numbers). One can then form the compound regular
-expression $(r_{key} + r_{id})^*$ and use it to tokenise strings. But
-then how should the string \textit{iffoo} be tokenised? It could be
-tokenised as a keyword followed by an identifier, or the entire string
-as a single identifier. Similarly, how should the string \textit{if} be
-tokenised? Both regular expressions, $r_{key}$ and $r_{id}$, would
-``fire''---so is it an identifier or a keyword? While in applications
-there is a well-known strategy to decide these questions, called POSIX
-matching, only relatively recently precise definitions of what POSIX
-matching actually means have been formalised
-\cite{AusafDyckhoffUrban2016,OkuiSuzuki2010,Vansummeren2006}. Such a
-definition has also been given by Sulzmann and Lu \cite{Sulzmann2014},
-but the corresponding correctness proof turned out to be faulty
-\cite{AusafDyckhoffUrban2016}. Roughly, POSIX matching means matching
-the longest initial substring. In the case of a tie, the initial
-sub-match is chosen according to some priorities attached to the regular
-expressions (e.g.~keywords have a higher priority than identifiers).
-This sounds rather simple, but according to Grathwohl et al \cite[Page
-36]{CrashCourse2014} this is not the case. They wrote:
-
-\begin{quote}
-\it{}``The POSIX strategy is more complicated than the greedy because of
-the dependence on information about the length of matched strings in the
-various subexpressions.''
-\end{quote}
-
-\noindent
-This is also supported by evidence collected by Kuklewicz
-\cite{Kuklewicz} who noticed that a number of POSIX regular expression
-matchers calculate incorrect results.
-
-Our focus in this project is on an algorithm introduced by Sulzmann and
-Lu in 2014 for regular expression matching according to the POSIX
-strategy \cite{Sulzmann2014}. Their algorithm is based on an older
-algorithm by Brzozowski from 1964 where he introduced the notion of
-derivatives of regular expressions~\cite{Brzozowski1964}. We shall
-briefly explain this algorithm next.
-
-\section{The Algorithm by Brzozowski based on Derivatives of Regular
-Expressions}
-
-Suppose (basic) regular expressions are given by the following grammar:
-\[ r ::= \ZERO \mid \ONE
- \mid c
- \mid r_1 \cdot r_2
- \mid r_1 + r_2
- \mid r^*
-\]
-
-\noindent
-The intended meaning of the constructors is as follows: $\ZERO$
-cannot match any string, $\ONE$ can match the empty string, the
-character regular expression $c$ can match the character $c$, and so
-on.
-
-The ingenious contribution by Brzozowski is the notion of
-\emph{derivatives} of regular expressions. The idea behind this
-notion is as follows: suppose a regular expression $r$ can match a
-string of the form $c\!::\! s$ (that is a list of characters starting
-with $c$), what does the regular expression look like that can match
-just $s$? Brzozowski gave a neat answer to this question. He started
-with the definition of $nullable$:
-\begin{center}
- \begin{tabular}{lcl}
- $\nullable(\ZERO)$ & $\dn$ & $\mathit{false}$ \\
- $\nullable(\ONE)$ & $\dn$ & $\mathit{true}$ \\
- $\nullable(c)$ & $\dn$ & $\mathit{false}$ \\
- $\nullable(r_1 + r_2)$ & $\dn$ & $\nullable(r_1) \vee \nullable(r_2)$ \\
- $\nullable(r_1\cdot r_2)$ & $\dn$ & $\nullable(r_1) \wedge \nullable(r_2)$ \\
- $\nullable(r^*)$ & $\dn$ & $\mathit{true}$ \\
- \end{tabular}
- \end{center}
-This function simply tests whether the empty string is in $L(r)$.
-He then defined
-the following operation on regular expressions, written
-$r\backslash c$ (the derivative of $r$ w.r.t.~the character $c$):
-
-\begin{center}
-\begin{tabular}{lcl}
- $\ZERO \backslash c$ & $\dn$ & $\ZERO$\\
- $\ONE \backslash c$ & $\dn$ & $\ZERO$\\
- $d \backslash c$ & $\dn$ &
- $\mathit{if} \;c = d\;\mathit{then}\;\ONE\;\mathit{else}\;\ZERO$\\
-$(r_1 + r_2)\backslash c$ & $\dn$ & $r_1 \backslash c \,+\, r_2 \backslash c$\\
-$(r_1 \cdot r_2)\backslash c$ & $\dn$ & $\mathit{if} \, nullable(r_1)$\\
- & & $\mathit{then}\;(r_1\backslash c) \cdot r_2 \,+\, r_2\backslash c$\\
- & & $\mathit{else}\;(r_1\backslash c) \cdot r_2$\\
- $(r^*)\backslash c$ & $\dn$ & $(r\backslash c) \cdot r^*$\\
-\end{tabular}
-\end{center}
-
-%Assuming the classic notion of a
-%\emph{language} of a regular expression, written $L(\_)$, t
-
-\noindent
-The main property of the derivative operation is that
-
-\begin{center}
-$c\!::\!s \in L(r)$ holds
-if and only if $s \in L(r\backslash c)$.
-\end{center}
-
-\noindent
-For us the main advantage is that derivatives can be
-straightforwardly implemented in any functional programming language,
-and are easily definable and reasoned about in theorem provers---the
-definitions just consist of inductive datatypes and simple recursive
-functions. Moreover, the notion of derivatives can be easily
-generalised to cover extended regular expression constructors such as
-the not-regular expression, written $\neg\,r$, or bounded repetitions
-(for example $r^{\{n\}}$ and $r^{\{n..m\}}$), which cannot be so
-straightforwardly realised within the classic automata approach.
-For the moment however, we focus only on the usual basic regular expressions.
-
-
-Now if we want to find out whether a string $s$ matches with a regular
-expression $r$, we can build the derivatives of $r$ w.r.t.\ (in succession)
-all the characters of the string $s$. Finally, test whether the
-resulting regular expression can match the empty string. If yes, then
-$r$ matches $s$, and no in the negative case. To implement this idea
-we can generalise the derivative operation to strings like this:
-
-\begin{center}
-\begin{tabular}{lcl}
-$r \backslash (c\!::\!s) $ & $\dn$ & $(r \backslash c) \backslash s$ \\
-$r \backslash [\,] $ & $\dn$ & $r$
-\end{tabular}
-\end{center}
-
-\noindent
-and then define as regular-expression matching algorithm:
-\[
-match\;s\;r \;\dn\; nullable(r\backslash s)
-\]
-
-\noindent
-This algorithm looks graphically as follows:
-\begin{equation}\label{graph:*}
-\begin{tikzcd}
-r_0 \arrow[r, "\backslash c_0"] & r_1 \arrow[r, "\backslash c_1"] & r_2 \arrow[r, dashed] & r_n \arrow[r,"\textit{nullable}?"] & \;\textrm{YES}/\textrm{NO}
-\end{tikzcd}
-\end{equation}
-
-\noindent
-where we start with a regular expression $r_0$, build successive
-derivatives until we exhaust the string and then use \textit{nullable}
-to test whether the result can match the empty string. It can be
-relatively easily shown that this matcher is correct (that is given
-an $s = c_0...c_{n-1}$ and an $r_0$, it generates YES if and only if $s \in L(r_0)$).
-
-
-\section{Values and the Algorithm by Sulzmann and Lu}
-
-One limitation of Brzozowski's algorithm is that it only produces a
-YES/NO answer for whether a string is being matched by a regular
-expression. Sulzmann and Lu~\cite{Sulzmann2014} extended this algorithm
-to allow generation of an actual matching, called a \emph{value} or
-sometimes also \emph{lexical value}. These values and regular
-expressions correspond to each other as illustrated in the following
-table:
-
-
-\begin{center}
- \begin{tabular}{c@{\hspace{20mm}}c}
- \begin{tabular}{@{}rrl@{}}
- \multicolumn{3}{@{}l}{\textbf{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}{@{}l}{\textbf{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
-No value corresponds to $\ZERO$; $\Empty$ corresponds to $\ONE$;
-$\Char$ to the character regular expression; $\Seq$ to the sequence
-regular expression and so on. The idea of values is to encode a kind of
-lexical value for how the sub-parts of a regular expression match the
-sub-parts of a string. To see this, suppose a \emph{flatten} operation,
-written $|v|$ for values. We can use this function to extract the
-underlying string of a value $v$. For example, $|\mathit{Seq} \,
-(\textit{Char x}) \, (\textit{Char y})|$ is the string $xy$. Using
-flatten, we can describe how values encode lexical values: $\Seq\,v_1\,
-v_2$ encodes a tree with two children nodes that tells how the string
-$|v_1| @ |v_2|$ matches the regex $r_1 \cdot r_2$ whereby $r_1$ matches
-the substring $|v_1|$ and, respectively, $r_2$ matches the substring
-$|v_2|$. Exactly how these two are matched is contained in the children
-nodes $v_1$ and $v_2$ of parent $\textit{Seq}$.
-
-To give a concrete example of how values work, consider the string $xy$
-and the regular expression $(x + (y + xy))^*$. We can view this regular
-expression as a tree and if the string $xy$ is matched by two Star
-``iterations'', then the $x$ is matched by the left-most alternative in
-this tree and the $y$ by the right-left alternative. This suggests to
-record this matching as
-
-\begin{center}
-$\Stars\,[\Left\,(\Char\,x), \Right(\Left(\Char\,y))]$
-\end{center}
-
-\noindent
-where $\Stars \; [\ldots]$ records all the
-iterations; and $\Left$, respectively $\Right$, which
-alternative is used. The value for
-matching $xy$ in a single ``iteration'', i.e.~the POSIX value,
-would look as follows
-
-\begin{center}
-$\Stars\,[\Seq\,(\Char\,x)\,(\Char\,y)]$
-\end{center}
-
-\noindent
-where $\Stars$ has only a single-element list for the single iteration
-and $\Seq$ indicates that $xy$ is matched by a sequence regular
-expression.
-
-The contribution of Sulzmann and Lu is an extension of Brzozowski's
-algorithm by a second phase (the first phase being building successive
-derivatives---see \eqref{graph:*}). In this second phase, a POSIX value
-is generated in case the regular expression matches the string.
-Pictorially, the Sulzmann and Lu algorithm is as follows:
-
-\begin{ceqn}
-\begin{equation}\label{graph:2}
-\begin{tikzcd}
-r_0 \arrow[r, "\backslash c_0"] \arrow[d] & r_1 \arrow[r, "\backslash c_1"] \arrow[d] & r_2 \arrow[r, dashed] \arrow[d] & r_n \arrow[d, "mkeps" description] \\
-v_0 & v_1 \arrow[l,"inj_{r_0} c_0"] & v_2 \arrow[l, "inj_{r_1} c_1"] & v_n \arrow[l, dashed]
-\end{tikzcd}
-\end{equation}
-\end{ceqn}
-
-\noindent
-For convenience, we shall employ the following notations: the regular
-expression we start with is $r_0$, and the given string $s$ is composed
-of characters $c_0 c_1 \ldots c_{n-1}$. In the first phase from the
-left to right, we build the derivatives $r_1$, $r_2$, \ldots according
-to the characters $c_0$, $c_1$ until we exhaust the string and obtain
-the derivative $r_n$. We test whether this derivative is
-$\textit{nullable}$ or not. If not, we know the string does not match
-$r$ and no value needs to be generated. If yes, we start building the
-values incrementally by \emph{injecting} back the characters into the
-earlier values $v_n, \ldots, v_0$. This is the second phase of the
-algorithm from the right to left. For the first value $v_n$, we call the
-function $\textit{mkeps}$, which builds the lexical value
-for how the empty string has been matched by the (nullable) regular
-expression $r_n$. This function is defined as
-
- \begin{center}
- \begin{tabular}{lcl}
- $\mkeps(\ONE)$ & $\dn$ & $\Empty$ \\
- $\mkeps(r_{1}+r_{2})$ & $\dn$
- & \textit{if} $\nullable(r_{1})$\\
- & & \textit{then} $\Left(\mkeps(r_{1}))$\\
- & & \textit{else} $\Right(\mkeps(r_{2}))$\\
- $\mkeps(r_1\cdot r_2)$ & $\dn$ & $\Seq\,(\mkeps\,r_1)\,(\mkeps\,r_2)$\\
- $mkeps(r^*)$ & $\dn$ & $\Stars\,[]$
- \end{tabular}
- \end{center}
-
-
-\noindent There 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 about what value is calculated.
-
-After the $\mkeps$-call, we inject back the characters one by one in order to build
-the lexical value $v_i$ for how the regex $r_i$ matches the string $s_i$
-($s_i = c_i \ldots c_{n-1}$ ) from the previous lexical value $v_{i+1}$.
-After injecting back $n$ characters, we get the lexical value for how $r_0$
-matches $s$. For this Sulzmann and Lu defined a function that reverses
-the ``chopping off'' of characters during the derivative phase. The
-corresponding function is called \emph{injection}, written
-$\textit{inj}$; it takes three arguments: the first one is a regular
-expression ${r_{i-1}}$, before the character is chopped off, the second
-is a character ${c_{i-1}}$, the character we want to inject and the
-third argument is the value ${v_i}$, into which one wants to inject the
-character (it corresponds to the regular expression after the character
-has been chopped off). 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 ``shape'' of regular
-expressions and values. To understands this definition better consider
-the situation when we build the derivative on regular expression $r_{i-1}$.
-For this we chop off a character from $r_{i-1}$ to form $r_i$. This leaves a
-``hole'' in $r_i$ and its corresponding value $v_i$.
-To calculate $v_{i-1}$, we need to
-locate where that hole is and fill it.
-We can find this location by
-comparing $r_{i-1}$ and $v_i$. For instance, if $r_{i-1}$ is of shape
-$r_a \cdot r_b$, and $v_i$ is of shape $\Left(Seq(v_1,v_2))$, we know immediately that
-%
-\[ (r_a \cdot r_b)\backslash c = (r_a\backslash c) \cdot r_b \,+\, r_b\backslash c,\]
-
-\noindent
-otherwise if $r_a$ is not nullable,
-\[ (r_a \cdot r_b)\backslash c = (r_a\backslash c) \cdot r_b,\]
-
-\noindent
-the value $v_i$ should be $\Seq(\ldots)$, contradicting the fact that
-$v_i$ is actually of shape $\Left(\ldots)$. Furthermore, since $v_i$ is of shape
-$\Left(\ldots)$ instead of $\Right(\ldots)$, we know that the left
-branch of \[ (r_a \cdot r_b)\backslash c =
-\bold{\underline{ (r_a\backslash c) \cdot r_b} }\,+\, r_b\backslash c,\](underlined)
- is taken instead of the right one. This means $c$ is chopped off
-from $r_a$ rather than $r_b$.
-We have therefore found out
-that the hole will be on $r_a$. So we recursively call $\inj\,
-r_a\,c\,v_a$ to fill that hole in $v_a$. After injection, the value
-$v_i$ for $r_i = r_a \cdot r_b$ should be $\Seq\,(\inj\,r_a\,c\,v_a)\,v_b$.
-Other clauses can be understood in a similar way.
-
-%\comment{Other word: insight?}
-The following example gives an insight of $\textit{inj}$'s effect and
-how Sulzmann and Lu's algorithm works as a whole. Suppose we have a
-regular expression $((((a+b)+ab)+c)+abc)^*$, and want to match it
-against the string $abc$ (when $abc$ is written as a regular expression,
-the standard way of expressing it is $a \cdot (b \cdot c)$. But we
-usually omit the parentheses and dots here for better readability. This
-algorithm returns a POSIX value, which means it will produce the longest
-matching. Consequently, it matches the string $abc$ in one star
-iteration, using the longest alternative $abc$ in the sub-expression (we shall use $r$ to denote this
-sub-expression for conciseness):
-
-\[((((a+b)+ab)+c)+\underbrace{abc}_r)\]
-
-\noindent
-Before $\textit{inj}$ is called, our lexer first builds derivative using
-string $abc$ (we simplified some regular expressions like $\ZERO \cdot
-b$ to $\ZERO$ for conciseness; we also omit parentheses if they are
-clear from the context):
-
-%Similarly, we allow
-%$\textit{ALT}$ to take a list of regular expressions as an argument
-%instead of just 2 operands to reduce the nested depth of
-%$\textit{ALT}$
-
-\begin{center}
-\begin{tabular}{lcl}
-$r^*$ & $\xrightarrow{\backslash a}$ & $r_1 = (\ONE+\ZERO+\ONE \cdot b + \ZERO + \ONE \cdot b \cdot c) \cdot r^*$\\
- & $\xrightarrow{\backslash b}$ & $r_2 = (\ZERO+\ZERO+\ONE \cdot \ONE + \ZERO + \ONE \cdot \ONE \cdot c) \cdot r^* +(\ZERO+\ONE+\ZERO + \ZERO + \ZERO) \cdot r^*$\\
- & $\xrightarrow{\backslash c}$ & $r_3 = ((\ZERO+\ZERO+\ZERO + \ZERO + \ONE \cdot \ONE \cdot \ONE) \cdot r^* + (\ZERO+\ZERO+\ZERO + \ONE + \ZERO) \cdot r^*) + $\\
- & & $\phantom{r_3 = (} ((\ZERO+\ONE+\ZERO + \ZERO + \ZERO) \cdot r^* + (\ZERO+\ZERO+\ZERO + \ONE + \ZERO) \cdot r^* )$
-\end{tabular}
-\end{center}
-
-\noindent
-In case $r_3$ is nullable, we can call $\textit{mkeps}$
-to construct a lexical value for how $r_3$ matched the string $abc$.
-This function gives the following value $v_3$:
-
-
-\begin{center}
-$\Left(\Left(\Seq(\Right(\Seq(\Empty, \Seq(\Empty,\Empty))), \Stars [])))$
-\end{center}
-The outer $\Left(\Left(\ldots))$ tells us the leftmost nullable part of $r_3$(underlined):
-
-\begin{center}
- \begin{tabular}{l@{\hspace{2mm}}l}
- & $\big(\underline{(\ZERO+\ZERO+\ZERO+ \ZERO+ \ONE \cdot \ONE \cdot \ONE) \cdot r^*}
- \;+\; (\ZERO+\ZERO+\ZERO + \ONE + \ZERO) \cdot r^*\big)$ \smallskip\\
- $+$ & $\big((\ZERO+\ONE+\ZERO + \ZERO + \ZERO) \cdot r^*
- \;+\; (\ZERO+\ZERO+\ZERO + \ONE + \ZERO) \cdot r^* \big)$
- \end{tabular}
- \end{center}
-
-\noindent
- Note that the leftmost location of term $(\ZERO+\ZERO+\ZERO + \ZERO + \ONE \cdot \ONE \cdot
- \ONE) \cdot r^*$ (which corresponds to the initial sub-match $abc$) allows
- $\textit{mkeps}$ to pick it up because $\textit{mkeps}$ is defined to always choose the
- left one when it is nullable. In the case of this example, $abc$ is
- preferred over $a$ or $ab$. This $\Left(\Left(\ldots))$ location is
- generated by two applications of the splitting clause
-
-\begin{center}
- $(r_1 \cdot r_2)\backslash c \;\;(when \; r_1 \; nullable) \, = (r_1\backslash c) \cdot r_2 \,+\, r_2\backslash c.$
-\end{center}
-
-\noindent
-By this clause, we put $r_1 \backslash c \cdot r_2 $ at the
-$\textit{front}$ and $r_2 \backslash c$ at the $\textit{back}$. This
-allows $\textit{mkeps}$ to always pick up among two matches the one with a longer
-initial sub-match. Removing the outside $\Left(\Left(...))$, the inside
-sub-value
-
-\begin{center}
- $\Seq(\Right(\Seq(\Empty, \Seq(\Empty, \Empty))), \Stars [])$
-\end{center}
-
-\noindent
-tells us how the empty string $[]$ is matched with $(\ZERO+\ZERO+\ZERO + \ZERO + \ONE \cdot
-\ONE \cdot \ONE) \cdot r^*$. We match $[]$ by a sequence of two nullable regular
-expressions. The first one is an alternative, we take the rightmost
-alternative---whose language contains the empty string. The second
-nullable regular expression is a Kleene star. $\Stars$ tells us how it
-generates the nullable regular expression: by 0 iterations to form
-$\ONE$. Now $\textit{inj}$ injects characters back and incrementally
-builds a lexical value based on $v_3$. Using the value $v_3$, the character
-c, and the regular expression $r_2$, we can recover how $r_2$ matched
-the string $[c]$ : $\textit{inj} \; r_2 \; c \; v_3$ gives us
- \begin{center}
- $v_2 = \Left(\Seq(\Right(\Seq(\Empty, \Seq(\Empty, c))), \Stars [])),$
- \end{center}
-which tells us how $r_2$ matched $[c]$. After this we inject back the character $b$, and get
-\begin{center}
-$v_1 = \Seq(\Right(\Seq(\Empty, \Seq(b, c))), \Stars [])$
-\end{center}
- for how
- \begin{center}
- $r_1= (\ONE+\ZERO+\ONE \cdot b + \ZERO + \ONE \cdot b \cdot c) \cdot r*$
- \end{center}
- matched the string $bc$ before it split into two substrings.
- Finally, after injecting character $a$ back to $v_1$,
- we get the lexical value tree
- \begin{center}
- $v_0= \Stars [\Right(\Seq(a, \Seq(b, c)))]$
- \end{center}
- for how $r$ matched $abc$. This completes the algorithm.
-
-%We omit the details of injection function, which is provided by Sulzmann and Lu's paper \cite{Sulzmann2014}.
-Readers might have noticed that the lexical value information is actually
-already available when doing derivatives. For example, immediately after
-the operation $\backslash a$ we know that if we want to match a string
-that starts with $a$, we can either take the initial match to be
-
- \begin{center}
-\begin{enumerate}
- \item[1)] just $a$ or
- \item[2)] string $ab$ or
- \item[3)] string $abc$.
-\end{enumerate}
-\end{center}
-
-\noindent
-In order to differentiate between these choices, we just need to
-remember their positions---$a$ is on the left, $ab$ is in the middle ,
-and $abc$ is on the right. Which of these alternatives is chosen
-later does not affect their relative position because the algorithm does
-not change this order. If this parsing information can be determined and
-does not change because of later derivatives, there is no point in
-traversing this information twice. This leads to an optimisation---if we
-store the information for lexical values inside the regular expression,
-update it when we do derivative on them, and collect the information
-when finished with derivatives and call $\textit{mkeps}$ for deciding which
-branch is POSIX, we can generate the lexical value in one pass, instead of
-doing the rest $n$ injections. This leads to Sulzmann and Lu's novel
-idea of using bitcodes in derivatives.
-
-In the next section, we shall focus on the bitcoded algorithm and the
-process of simplification of regular expressions. This is needed in
-order to obtain \emph{fast} versions of the Brzozowski's, and Sulzmann
-and Lu's algorithms. This is where the PhD-project aims to advance the
-state-of-the-art.
-
-
-\section{Simplification of Regular Expressions}
-
-Using bitcodes to guide parsing is not a novel idea. It was applied to
-context free grammars and then adapted by Henglein and Nielson for
-efficient regular expression lexing using DFAs~\cite{nielson11bcre}.
-Sulzmann and Lu took this idea of bitcodes a step further by integrating
-bitcodes into derivatives. The reason why we want to use bitcodes in
-this project is that we want to introduce more aggressive simplification
-rules in order to keep the size of derivatives small throughout. This is
-because the main drawback of building successive derivatives according
-to Brzozowski's definition is that they can grow very quickly in size.
-This is mainly due to the fact that the derivative operation generates
-often ``useless'' $\ZERO$s and $\ONE$s in derivatives. As a result, if
-implemented naively both algorithms by Brzozowski and by Sulzmann and Lu
-are excruciatingly slow. For example when starting with the regular
-expression $(a + aa)^*$ and building 12 successive derivatives
-w.r.t.~the character $a$, one obtains a derivative regular expression
-with more than 8000 nodes (when viewed as a tree). Operations like
-$\textit{der}$ and $\nullable$ need to traverse such trees and
-consequently the bigger the size of the derivative the slower the
-algorithm.
-
-Fortunately, one can simplify regular expressions after each derivative
-step. Various simplifications of regular expressions are possible, such
-as the simplification of $\ZERO + r$, $r + \ZERO$, $\ONE\cdot r$, $r
-\cdot \ONE$, and $r + r$ to just $r$. These simplifications do not
-affect the answer for whether a regular expression matches a string or
-not, but fortunately also do not affect the POSIX strategy of how
-regular expressions match strings---although the latter is much harder
-to establish. Some initial results in this regard have been
-obtained in \cite{AusafDyckhoffUrban2016}.
-
-Unfortunately, the simplification rules outlined above are not
-sufficient to prevent a size explosion in all cases. We
-believe a tighter bound can be achieved that prevents an explosion in
-\emph{all} cases. Such a tighter bound is suggested by work of Antimirov who
-proved that (partial) derivatives can be bound by the number of
-characters contained in the initial regular expression
-\cite{Antimirov95}. He defined the \emph{partial derivatives} of regular
-expressions as follows:
-
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{pder} \; c \; \ZERO$ & $\dn$ & $\emptyset$\\
- $\textit{pder} \; c \; \ONE$ & $\dn$ & $\emptyset$ \\
- $\textit{pder} \; c \; d$ & $\dn$ & $\textit{if} \; c \,=\, d \; \{ \ONE \} \; \textit{else} \; \emptyset$ \\
- $\textit{pder} \; c \; r_1+r_2$ & $\dn$ & $pder \; c \; r_1 \cup pder \; c \; r_2$ \\
- $\textit{pder} \; c \; r_1 \cdot r_2$ & $\dn$ & $\textit{if} \; nullable \; r_1 $\\
- & & $\textit{then} \; \{ r \cdot r_2 \mid r \in pder \; c \; r_1 \} \cup pder \; c \; r_2 \;$\\
- & & $\textit{else} \; \{ r \cdot r_2 \mid r \in pder \; c \; r_1 \} $ \\
- $\textit{pder} \; c \; r^*$ & $\dn$ & $ \{ r' \cdot r^* \mid r' \in pder \; c \; r \} $ \\
- \end{tabular}
- \end{center}
-
-\noindent
-A partial derivative of a regular expression $r$ is essentially a set of
-regular expressions that are either $r$'s children expressions or a
-concatenation of them. Antimirov has proved a tight bound of the sum of
-the size of \emph{all} partial derivatives no matter what the string
-looks like. Roughly speaking the size sum will be at most cubic in the
-size of the regular expression.
-
-If we want the size of derivatives in Sulzmann and Lu's algorithm to
-stay below this bound, we would need more aggressive simplifications.
-Essentially we need to delete useless $\ZERO$s and $\ONE$s, as well as
-deleting duplicates whenever possible. For example, the parentheses in
-$(a+b) \cdot c + bc$ can be opened up to get $a\cdot c + b \cdot c + b
-\cdot c$, and then simplified to just $a \cdot c + b \cdot c$. Another
-example is simplifying $(a^*+a) + (a^*+ \ONE) + (a +\ONE)$ to just
-$a^*+a+\ONE$. Adding these more aggressive simplification rules helps us
-to achieve the same size bound as that of the partial derivatives.
-
-In order to implement the idea of ``spilling out alternatives'' and to
-make them compatible with the $\text{inj}$-mechanism, we use
-\emph{bitcodes}. Bits and bitcodes (lists of bits) are just:
-
-%This allows us to prove a tight
-%bound on the size of regular expression during the running time of the
-%algorithm if we can establish the connection between our simplification
-%rules and partial derivatives.
-
- %We believe, and have generated test
-%data, that a similar bound can be obtained for the derivatives in
-%Sulzmann and Lu's algorithm. Let us give some details about this next.
-
-
-\begin{center}
- $b ::= 0 \mid 1 \qquad
-bs ::= [] \mid b:bs
-$
-\end{center}
-
-\noindent
-The $0$ and $1$ are arbitrary names for the bits and not in bold
-in order to avoid
-confusion with the regular expressions $\ZERO$ and $\ONE$. Bitcodes (or
-bit-lists) can be used to encode values (or incomplete values) in a
-compact form. This can be straightforwardly seen in the following
-coding function from values to bitcodes:
-
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{code}(\Empty)$ & $\dn$ & $[]$\\
- $\textit{code}(\Char\,c)$ & $\dn$ & $[]$\\
- $\textit{code}(\Left\,v)$ & $\dn$ & $0 :: code(v)$\\
- $\textit{code}(\Right\,v)$ & $\dn$ & $1 :: code(v)$\\
- $\textit{code}(\Seq\,v_1\,v_2)$ & $\dn$ & $code(v_1) \,@\, code(v_2)$\\
- $\textit{code}(\Stars\,[])$ & $\dn$ & $[0]$\\
- $\textit{code}(\Stars\,(v\!::\!vs))$ & $\dn$ & $1 :: code(v) \;@\;
- code(\Stars\,vs)$
-\end{tabular}
-\end{center}
-
-\noindent
-Here $\textit{code}$ encodes a value into a bitcodes by converting
-$\Left$ into $0$, $\Right$ into $1$, the start point of a non-empty
-star iteration into $\S$, and the border where a local star terminates
-into $0$. This coding is lossy, as it throws away the information about
-characters, and also does not encode the ``boundary'' between two
-sequence values. Moreover, with only the bitcode we cannot even tell
-whether the $1$s and $0$s are for $\Left/\Right$ or $\Stars$. The
-reason for choosing this compact way of storing information is that the
-relatively small size of bits can be easily manipulated and ``moved
-around'' in a regular expression. In order to recover values, we will
-need the corresponding regular expression as an extra information. This
-means the decoding function is defined as:
-
-
-%\begin{definition}[Bitdecoding of Values]\mbox{}
-\begin{center}
-\begin{tabular}{@{}l@{\hspace{1mm}}c@{\hspace{1mm}}l@{}}
- $\textit{decode}'\,bs\,(\ONE)$ & $\dn$ & $(\Empty, bs)$\\
- $\textit{decode}'\,bs\,(c)$ & $\dn$ & $(\Char\,c, bs)$\\
- $\textit{decode}'\,(0\!::\!bs)\;(r_1 + r_2)$ & $\dn$ &
- $\textit{let}\,(v, bs_1) = \textit{decode}'\,bs\,r_1\;\textit{in}\;
- (\Left\,v, bs_1)$\\
- $\textit{decode}'\,(1\!::\!bs)\;(r_1 + r_2)$ & $\dn$ &
- $\textit{let}\,(v, bs_1) = \textit{decode}'\,bs\,r_2\;\textit{in}\;
- (\Right\,v, bs_1)$\\
- $\textit{decode}'\,bs\;(r_1\cdot r_2)$ & $\dn$ &
- $\textit{let}\,(v_1, bs_1) = \textit{decode}'\,bs\,r_1\;\textit{in}$\\
- & & $\textit{let}\,(v_2, bs_2) = \textit{decode}'\,bs_1\,r_2$\\
- & & \hspace{35mm}$\textit{in}\;(\Seq\,v_1\,v_2, bs_2)$\\
- $\textit{decode}'\,(0\!::\!bs)\,(r^*)$ & $\dn$ & $(\Stars\,[], bs)$\\
- $\textit{decode}'\,(1\!::\!bs)\,(r^*)$ & $\dn$ &
- $\textit{let}\,(v, bs_1) = \textit{decode}'\,bs\,r\;\textit{in}$\\
- & & $\textit{let}\,(\Stars\,vs, bs_2) = \textit{decode}'\,bs_1\,r^*$\\
- & & \hspace{35mm}$\textit{in}\;(\Stars\,v\!::\!vs, bs_2)$\bigskip\\
-
- $\textit{decode}\,bs\,r$ & $\dn$ &
- $\textit{let}\,(v, bs') = \textit{decode}'\,bs\,r\;\textit{in}$\\
- & & $\textit{if}\;bs' = []\;\textit{then}\;\textit{Some}\,v\;
- \textit{else}\;\textit{None}$
-\end{tabular}
-\end{center}
-%\end{definition}
-
-Sulzmann and Lu's integrated the bitcodes into regular expressions to
-create annotated regular expressions \cite{Sulzmann2014}.
-\emph{Annotated regular expressions} are defined by the following
-grammar:%\comment{ALTS should have an $as$ in the definitions, not just $a_1$ and $a_2$}
-
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{a}$ & $::=$ & $\ZERO$\\
- & $\mid$ & $_{bs}\ONE$\\
- & $\mid$ & $_{bs}{\bf c}$\\
- & $\mid$ & $_{bs}\oplus \textit{as}$\\
- & $\mid$ & $_{bs}a_1\cdot a_2$\\
- & $\mid$ & $_{bs}a^*$
-\end{tabular}
-\end{center}
-%(in \textit{ALTS})
-
-\noindent
-where $\textit{bs}$ stands for bitcodes, $a$ for $\textit{annotated}$ regular
-expressions and $\textit{as}$ for a list of annotated regular expressions.
-The alternative constructor($\oplus$) has been generalized to
-accept a list of annotated regular expressions rather than just 2.
-We will show that these bitcodes encode information about
-the (POSIX) value that should be generated by the Sulzmann and Lu
-algorithm.
-
-
-To do lexing using annotated regular expressions, we shall first
-transform the usual (un-annotated) regular expressions into annotated
-regular expressions. This operation is called \emph{internalisation} and
-defined as follows:
-
-%\begin{definition}
-\begin{center}
-\begin{tabular}{lcl}
- $(\ZERO)^\uparrow$ & $\dn$ & $\ZERO$\\
- $(\ONE)^\uparrow$ & $\dn$ & $_{[]}\ONE$\\
- $(c)^\uparrow$ & $\dn$ & $_{bs}\textit{\bf c}$\\
- $(r_1 + r_2)^\uparrow$ & $\dn$ &
- $_{[]}\oplus\,[(\textit{fuse}\,[0]\,r_1^\uparrow),\,
- (\textit{fuse}\,[1]\,r_2^\uparrow)]$\\
- $(r_1\cdot r_2)^\uparrow$ & $\dn$ &
- $_{[]}r_1^\uparrow \cdot r_2^\uparrow$\\
- $(r^*)^\uparrow$ & $\dn$ &
- $_{[]}r^\uparrow$\\
-\end{tabular}
-\end{center}
-%\end{definition}
-
-\noindent
-We use up arrows here to indicate that the basic un-annotated regular
-expressions are ``lifted up'' into something slightly more complex. In the
-fourth clause, $\textit{fuse}$ is an auxiliary function that helps to
-attach bits to the front of an annotated regular expression. Its
-definition is as follows:
-
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{fuse}\;bs\,(\ZERO)$ & $\dn$ & $\textit{ZERO}$\\
- $\textit{fuse}\;bs\,(\textit{ONE}\,bs')$ & $\dn$ &
- $\textit{ONE}\,(bs\,@\,bs')$\\
- $\textit{fuse}\;bs\,(\textit{CHAR}\,bs'\,c)$ & $\dn$ &
- $\textit{CHAR}\,(bs\,@\,bs')\,c$\\
- $\textit{fuse}\;bs\,(\textit{ALTS}\,bs'\,as)$ & $\dn$ &
- $\textit{ALTS}\,(bs\,@\,bs')\,as$\\
- $\textit{fuse}\;bs\,(\textit{SEQ}\,bs'\,a_1\,a_2)$ & $\dn$ &
- $\textit{SEQ}\,(bs\,@\,bs')\,a_1\,a_2$\\
- $\textit{fuse}\;bs\,(\textit{STAR}\,bs'\,a)$ & $\dn$ &
- $\textit{STAR}\,(bs\,@\,bs')\,a$
-\end{tabular}
-\end{center}
-
-\noindent
-After internalising the regular expression, we perform successive
-derivative operations on the annotated regular expressions. This
-derivative operation is the same as what we had previously for the
-basic regular expressions, except that we beed to take care of
-the bitcodes:
-
- %\begin{definition}{bder}
-\begin{center}
- \begin{tabular}{@{}lcl@{}}
- $(\textit{ZERO})\,\backslash c$ & $\dn$ & $\textit{ZERO}$\\
- $(\textit{ONE}\;bs)\,\backslash c$ & $\dn$ & $\textit{ZERO}$\\
- $(\textit{CHAR}\;bs\,d)\,\backslash c$ & $\dn$ &
- $\textit{if}\;c=d\; \;\textit{then}\;
- \textit{ONE}\;bs\;\textit{else}\;\textit{ZERO}$\\
- $(\textit{ALTS}\;bs\,as)\,\backslash c$ & $\dn$ &
- $\textit{ALTS}\;bs\,(as.map(\backslash c))$\\
- $(\textit{SEQ}\;bs\,a_1\,a_2)\,\backslash c$ & $\dn$ &
- $\textit{if}\;\textit{bnullable}\,a_1$\\
- & &$\textit{then}\;\textit{ALTS}\,bs\,List((\textit{SEQ}\,[]\,(a_1\,\backslash c)\,a_2),$\\
- & &$\phantom{\textit{then}\;\textit{ALTS}\,bs\,}(\textit{fuse}\,(\textit{bmkeps}\,a_1)\,(a_2\,\backslash c)))$\\
- & &$\textit{else}\;\textit{SEQ}\,bs\,(a_1\,\backslash c)\,a_2$\\
- $(\textit{STAR}\,bs\,a)\,\backslash c$ & $\dn$ &
- $\textit{SEQ}\;bs\,(\textit{fuse}\, [\Z] (r\,\backslash c))\,
- (\textit{STAR}\,[]\,r)$
-\end{tabular}
-\end{center}
-%\end{definition}
-
-\noindent
-For instance, when we unfold $\textit{STAR} \; bs \; a$ into a sequence,
-we need to attach an additional bit $Z$ to the front of $r \backslash c$
-to indicate that there is one more star iteration. Also the $SEQ$ clause
-is more subtle---when $a_1$ is $\textit{bnullable}$ (here
-\textit{bnullable} is exactly the same as $\textit{nullable}$, except
-that it is for annotated regular expressions, therefore we omit the
-definition). Assume that $bmkeps$ correctly extracts the bitcode for how
-$a_1$ matches the string prior to character $c$ (more on this later),
-then the right branch of $ALTS$, which is $fuse \; bmkeps \; a_1 (a_2
-\backslash c)$ will collapse the regular expression $a_1$(as it has
-already been fully matched) and store the parsing information at the
-head of the regular expression $a_2 \backslash c$ by fusing to it. The
-bitsequence $bs$, which was initially attached to the head of $SEQ$, has
-now been elevated to the top-level of $ALTS$, as this information will be
-needed whichever way the $SEQ$ is matched---no matter whether $c$ belongs
-to $a_1$ or $ a_2$. After building these derivatives and maintaining all
-the lexing information, we complete the lexing by collecting the
-bitcodes using a generalised version of the $\textit{mkeps}$ function
-for annotated regular expressions, called $\textit{bmkeps}$:
-
-
-%\begin{definition}[\textit{bmkeps}]\mbox{}
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{bmkeps}\,(\textit{ONE}\;bs)$ & $\dn$ & $bs$\\
- $\textit{bmkeps}\,(\textit{ALTS}\;bs\,a::as)$ & $\dn$ &
- $\textit{if}\;\textit{bnullable}\,a$\\
- & &$\textit{then}\;bs\,@\,\textit{bmkeps}\,a$\\
- & &$\textit{else}\;bs\,@\,\textit{bmkeps}\,(\textit{ALTS}\;bs\,as)$\\
- $\textit{bmkeps}\,(\textit{SEQ}\;bs\,a_1\,a_2)$ & $\dn$ &
- $bs \,@\,\textit{bmkeps}\,a_1\,@\, \textit{bmkeps}\,a_2$\\
- $\textit{bmkeps}\,(\textit{STAR}\;bs\,a)$ & $\dn$ &
- $bs \,@\, [\S]$
-\end{tabular}
-\end{center}
-%\end{definition}
-
-\noindent
-This function completes the value information by travelling along the
-path of the regular expression that corresponds to a POSIX value and
-collecting all the bitcodes, and using $S$ to indicate the end of star
-iterations. If we take the bitcodes produced by $\textit{bmkeps}$ and
-decode them, we get the value we expect. The corresponding lexing
-algorithm looks as follows:
-
-\begin{center}
-\begin{tabular}{lcl}
- $\textit{blexer}\;r\,s$ & $\dn$ &
- $\textit{let}\;a = (r^\uparrow)\backslash s\;\textit{in}$\\
- & & $\;\;\textit{if}\; \textit{bnullable}(a)$\\
- & & $\;\;\textit{then}\;\textit{decode}\,(\textit{bmkeps}\,a)\,r$\\
- & & $\;\;\textit{else}\;\textit{None}$
-\end{tabular}
-\end{center}
-
-\noindent
-In this definition $\_\backslash s$ is the generalisation of the derivative
-operation from characters to strings (just like the derivatives for un-annotated
-regular expressions).
-
-The main point of the bitcodes and annotated regular expressions is that
-we can apply rather aggressive (in terms of size) simplification rules
-in order to keep derivatives small. We have developed such
-``aggressive'' simplification rules and generated test data that show
-that the expected bound can be achieved. Obviously we could only
-partially cover the search space as there are infinitely many regular
-expressions and strings.
-
-One modification we introduced is to allow a list of annotated regular
-expressions in the \textit{ALTS} constructor. This allows us to not just
-delete unnecessary $\ZERO$s and $\ONE$s from regular expressions, but
-also unnecessary ``copies'' of regular expressions (very similar to
-simplifying $r + r$ to just $r$, but in a more general setting). Another
-modification is that we use simplification rules inspired by Antimirov's
-work on partial derivatives. They maintain the idea that only the first
-``copy'' of a regular expression in an alternative contributes to the
-calculation of a POSIX value. All subsequent copies can be pruned away from
-the regular expression. A recursive definition of our simplification function
-that looks somewhat similar to our Scala code is given below:
-%\comment{Use $\ZERO$, $\ONE$ and so on.
-%Is it $ALTS$ or $ALTS$?}\\
-
-\begin{center}
- \begin{tabular}{@{}lcl@{}}
-
- $\textit{simp} \; (\textit{SEQ}\;bs\,a_1\,a_2)$ & $\dn$ & $ (\textit{simp} \; a_1, \textit{simp} \; a_2) \; \textit{match} $ \\
- &&$\quad\textit{case} \; (\ZERO, \_) \Rightarrow \ZERO$ \\
- &&$\quad\textit{case} \; (\_, \ZERO) \Rightarrow \ZERO$ \\
- &&$\quad\textit{case} \; (\ONE, a_2') \Rightarrow \textit{fuse} \; bs \; a_2'$ \\
- &&$\quad\textit{case} \; (a_1', \ONE) \Rightarrow \textit{fuse} \; bs \; a_1'$ \\
- &&$\quad\textit{case} \; (a_1', a_2') \Rightarrow \textit{SEQ} \; bs \; a_1' \; a_2'$ \\
-
- $\textit{simp} \; (\textit{ALTS}\;bs\,as)$ & $\dn$ & $\textit{distinct}( \textit{flatten} ( \textit{map simp as})) \; \textit{match} $ \\
- &&$\quad\textit{case} \; [] \Rightarrow \ZERO$ \\
- &&$\quad\textit{case} \; a :: [] \Rightarrow \textit{fuse bs a}$ \\
- &&$\quad\textit{case} \; as' \Rightarrow \textit{ALTS}\;bs\;as'$\\
-
- $\textit{simp} \; a$ & $\dn$ & $\textit{a} \qquad \textit{otherwise}$
-\end{tabular}
-\end{center}
-
-\noindent
-The simplification does a pattern matching on the regular expression.
-When it detected that the regular expression is an alternative or
-sequence, it will try to simplify its children regular expressions
-recursively and then see if one of the children turn into $\ZERO$ or
-$\ONE$, which might trigger further simplification at the current level.
-The most involved part is the $\textit{ALTS}$ clause, where we use two
-auxiliary functions $\textit{flatten}$ and $\textit{distinct}$ to open up nested
-$\textit{ALTS}$ and reduce as many duplicates as possible. Function
-$\textit{distinct}$ keeps the first occurring copy only and remove all later ones
-when detected duplicates. Function $\textit{flatten}$ opens up nested \textit{ALTS}.
-Its recursive definition is given below:
-
- \begin{center}
- \begin{tabular}{@{}lcl@{}}
- $\textit{flatten} \; (\textit{ALTS}\;bs\,as) :: as'$ & $\dn$ & $(\textit{map} \;
- (\textit{fuse}\;bs)\; \textit{as}) \; @ \; \textit{flatten} \; as' $ \\
- $\textit{flatten} \; \textit{ZERO} :: as'$ & $\dn$ & $ \textit{flatten} \; as' $ \\
- $\textit{flatten} \; a :: as'$ & $\dn$ & $a :: \textit{flatten} \; as'$ \quad(otherwise)
-\end{tabular}
-\end{center}
-
-\noindent
-Here $\textit{flatten}$ behaves like the traditional functional programming flatten
-function, except that it also removes $\ZERO$s. Or in terms of regular expressions, it
-removes parentheses, for example changing $a+(b+c)$ into $a+b+c$.
-
-Suppose we apply simplification after each derivative step, and view
-these two operations as an atomic one: $a \backslash_{simp}\,c \dn
-\textit{simp}(a \backslash c)$. Then we can use the previous natural
-extension from derivative w.r.t.~character to derivative
-w.r.t.~string:%\comment{simp in the [] case?}
-
-\begin{center}
-\begin{tabular}{lcl}
-$r \backslash_{simp} (c\!::\!s) $ & $\dn$ & $(r \backslash_{simp}\, c) \backslash_{simp}\, s$ \\
-$r \backslash_{simp} [\,] $ & $\dn$ & $r$
-\end{tabular}
-\end{center}
-
-\noindent
-we obtain an optimised version of the algorithm:
-
- \begin{center}
-\begin{tabular}{lcl}
- $\textit{blexer\_simp}\;r\,s$ & $\dn$ &
- $\textit{let}\;a = (r^\uparrow)\backslash_{simp}\, s\;\textit{in}$\\
- & & $\;\;\textit{if}\; \textit{bnullable}(a)$\\
- & & $\;\;\textit{then}\;\textit{decode}\,(\textit{bmkeps}\,a)\,r$\\
- & & $\;\;\textit{else}\;\textit{None}$
-\end{tabular}
-\end{center}
-
-\noindent
-This algorithm keeps the regular expression size small, for example,
-with this simplification our previous $(a + aa)^*$ example's 8000 nodes
-will be reduced to just 6 and stays constant, no matter how long the
-input string is.
-\section{Current Work}
-We are currently engaged in two tasks related to this algorithm. The
-first task is proving that our simplification rules actually do not
-affect the POSIX value that should be generated by the algorithm
-according to the specification of a POSIX value and furthermore obtain a
-much tighter bound on the sizes of derivatives. The result is that our
-algorithm should be correct and faster on all inputs. The original
-blow-up, as observed in JavaScript, Python and Java, would be excluded
-from happening in our algorithm. For this proof we use the theorem prover
-Isabelle. Once completed, this result will advance the state-of-the-art:
-Sulzmann and Lu wrote in their paper~\cite{Sulzmann2014} about the
-bitcoded ``incremental parsing method'' (that is the lexing algorithm
-outlined in this section):
-
-\begin{quote}\it
- ``Correctness Claim: We further claim that the incremental parsing
- method in Figure~5 in combination with the simplification steps in
- Figure 6 yields POSIX parse tree [our lexical values]. We have tested this claim
- extensively by using the method in Figure~3 as a reference but yet
- have to work out all proof details.''
-\end{quote}
-
-\noindent
-We like to settle this correctness claim. It is relatively
-straightforward to establish that after one simplification step, the part of a
-nullable derivative that corresponds to a POSIX value remains intact and can
-still be collected, in other words, we can show that
-%\comment{Double-check....I
-%think this is not the case}
-%\comment{If i remember correctly, you have proved this lemma.
-%I feel this is indeed not true because you might place arbitrary
-%bits on the regex r, however if this is the case, did i remember wrongly that
-%you proved something like simplification does not affect $\textit{bmkeps}$ results?
-%Anyway, i have amended this a little bit so it does not allow arbitrary bits attached
-%to a regex. Maybe it works now.}
-
-\begin{center}
- $\textit{bmkeps} \; a = \textit{bmkeps} \; \textit{bsimp} \; a\;($\textit{provided}$ \; a\; is \; \textit{bnullable} )$
-\end{center}
-
-\noindent
-as this basically comes down to proving actions like removing the
-additional $r$ in $r+r$ does not delete important POSIX information in
-a regular expression. The hard part of this proof is to establish that
-
-\begin{center}
- $ \textit{blexer}\_{simp}(r, \; s) = \textit{blexer}(r, \; s)$
-\end{center}
-%comment{This is not true either...look at the definion blexer/blexer-simp}
-
-\noindent That is, if we do derivative on regular expression $r$ and then
-simplify it, and repeat this process until we exhaust the string, we get a
-regular expression $r''$($\textit{LHS}$) that provides the POSIX matching
-information, which is exactly the same as the result $r'$($\textit{RHS}$ of the
-normal derivative algorithm that only does derivative repeatedly and has no
-simplification at all. This might seem at first glance very unintuitive, as
-the $r'$ could be exponentially larger than $r''$, but can be explained in the
-following way: we are pruning away the possible matches that are not POSIX.
-Since there could be exponentially many
-non-POSIX matchings and only 1 POSIX matching, it
-is understandable that our $r''$ can be a lot smaller. we can still provide
-the same POSIX value if there is one. This is not as straightforward as the
-previous proposition, as the two regular expressions $r'$ and $r''$ might have
-become very different. The crucial point is to find the
-$\textit{POSIX}$ information of a regular expression and how it is modified,
-augmented and propagated
-during simplification in parallel with the regular expression that
-has not been simplified in the subsequent derivative operations. To aid this,
-we use the helper function retrieve described by Sulzmann and Lu:
-\begin{center}
-\begin{tabular}{@{}l@{\hspace{2mm}}c@{\hspace{2mm}}l@{}}
- $\textit{retrieve}\,(\textit{ONE}\,bs)\,\Empty$ & $\dn$ & $bs$\\
- $\textit{retrieve}\,(\textit{CHAR}\,bs\,c)\,(\Char\,d)$ & $\dn$ & $bs$\\
- $\textit{retrieve}\,(\textit{ALTS}\,bs\,a::as)\,(\Left\,v)$ & $\dn$ &
- $bs \,@\, \textit{retrieve}\,a\,v$\\
- $\textit{retrieve}\,(\textit{ALTS}\,bs\,a::as)\,(\Right\,v)$ & $\dn$ &
- $bs \,@\, \textit{retrieve}\,(\textit{ALTS}\,bs\,as)\,v$\\
- $\textit{retrieve}\,(\textit{SEQ}\,bs\,a_1\,a_2)\,(\Seq\,v_1\,v_2)$ & $\dn$ &
- $bs \,@\,\textit{retrieve}\,a_1\,v_1\,@\, \textit{retrieve}\,a_2\,v_2$\\
- $\textit{retrieve}\,(\textit{STAR}\,bs\,a)\,(\Stars\,[])$ & $\dn$ &
- $bs \,@\, [\S]$\\
- $\textit{retrieve}\,(\textit{STAR}\,bs\,a)\,(\Stars\,(v\!::\!vs))$ & $\dn$ &\\
- \multicolumn{3}{l}{
- \hspace{3cm}$bs \,@\, [\Z] \,@\, \textit{retrieve}\,a\,v\,@\,
- \textit{retrieve}\,(\textit{STAR}\,[]\,a)\,(\Stars\,vs)$}\\
-\end{tabular}
-\end{center}
-%\comment{Did not read further}\\
-This function assembles the bitcode
-%that corresponds to a lexical value for how
-%the current derivative matches the suffix of the string(the characters that
-%have not yet appeared, but will appear as the successive derivatives go on.
-%How do we get this "future" information? By the value $v$, which is
-%computed by a pass of the algorithm that uses
-%$inj$ as described in the previous section).
-using information from both the derivative regular expression and the
-value. Sulzmann and Lu poroposed this function, but did not prove
-anything about it. Ausaf and Urban used it to connect the bitcoded
-algorithm to the older algorithm by the following equation:
-
- \begin{center} $inj \;a\; c \; v = \textit{decode} \; (\textit{retrieve}\;
- (r^\uparrow)\backslash_{simp} \,c)\,v)$
- \end{center}
-
-\noindent
-whereby $r^\uparrow$ stands for the internalised version of $r$. Ausaf
-and Urban also used this fact to prove the correctness of bitcoded
-algorithm without simplification. Our purpose of using this, however,
-is to establish
-
-\begin{center}
-$ \textit{retrieve} \;
-a \; v \;=\; \textit{retrieve} \; (\textit{simp}\,a) \; v'.$
-\end{center}
-The idea is that using $v'$, a simplified version of $v$ that had gone
-through the same simplification step as $\textit{simp}(a)$, we are able
-to extract the bitcode that gives the same parsing information as the
-unsimplified one. However, we noticed that constructing such a $v'$
-from $v$ is not so straightforward. The point of this is that we might
-be able to finally bridge the gap by proving
-
-\begin{center}
-$\textit{retrieve} \; (r^\uparrow \backslash s) \; v = \;\textit{retrieve} \;
-(\textit{simp}(r^\uparrow) \backslash s) \; v'$
-\end{center}
-
-\noindent
-and subsequently
-
-\begin{center}
-$\textit{retrieve} \; (r^\uparrow \backslash s) \; v\; = \; \textit{retrieve} \;
-(r^\uparrow \backslash_{simp} \, s) \; v'$.
-\end{center}
-
-\noindent
-The $\textit{LHS}$ of the above equation is the bitcode we want. This
-would prove that our simplified version of regular expression still
-contains all the bitcodes needed. The task here is to find a way to
-compute the correct $v'$.
-
-The second task is to speed up the more aggressive simplification. Currently
-it is slower than the original naive simplification by Ausaf and Urban (the
-naive version as implemented by Ausaf and Urban of course can ``explode'' in
-some cases). It is therefore not surprising that the speed is also much slower
-than regular expression engines in popular programming languages such as Java
-and Python on most inputs that are linear. For example, just by rewriting the
-example regular expression in the beginning of this report $(a^*)^*\,b$ into
-$a^*\,b$ would eliminate the ambiguity in the matching and make the time
-for matching linear with respect to the input string size. This allows the
-DFA approach to become blindingly fast, and dwarf the speed of our current
-implementation. For example, here is a comparison of Java regex engine
-and our implementation on this example.
-
-\begin{center}
-\begin{tabular}{@{}c@{\hspace{0mm}}c@{\hspace{0mm}}c@{}}
-\begin{tikzpicture}
-\begin{axis}[
- xlabel={$n*1000$},
- x label style={at={(1.05,-0.05)}},
- ylabel={time in secs},
- enlargelimits=false,
- xtick={0,5,...,30},
- xmax=33,
- ymax=9,
- scaled ticks=true,
- axis lines=left,
- width=5cm,
- height=4cm,
- legend entries={Bitcoded Algorithm},
- legend pos=north west,
- legend cell align=left]
-\addplot[red,mark=*, mark options={fill=white}] table {bad-scala.data};
-\end{axis}
-\end{tikzpicture}
- &
-\begin{tikzpicture}
-\begin{axis}[
- xlabel={$n*1000$},
- x label style={at={(1.05,-0.05)}},
- %ylabel={time in secs},
- enlargelimits=false,
- xtick={0,5,...,30},
- xmax=33,
- ymax=9,
- scaled ticks=false,
- axis lines=left,
- width=5cm,
- height=4cm,
- legend entries={Java},
- legend pos=north west,
- legend cell align=left]
-\addplot[cyan,mark=*, mark options={fill=white}] table {good-java.data};
-\end{axis}
-\end{tikzpicture}\\
-\multicolumn{3}{c}{Graphs: Runtime for matching $a^*\,b$ with strings
- of the form $\underbrace{aa..a}_{n}$.}
-\end{tabular}
-\end{center}
-
-
-Java regex engine can match string of thousands of characters in a few milliseconds,
-whereas our current algorithm gets excruciatingly slow on input of this size.
-The running time in theory is linear, however it does not appear to be the
-case in an actual implementation. So it needs to be explored how to
-make our algorithm faster on all inputs. It could be the recursive calls that are
-needed to manipulate bits that are causing the slow down. A possible solution
-is to write recursive functions into tail-recusive form.
-Another possibility would be to explore
-again the connection to DFAs to speed up the algorithm on
-subcalls that are small enough. This is very much work in progress.
-
-\section{Conclusion}
-
-In this PhD-project we are interested in fast algorithms for regular
-expression matching. While this seems to be a ``settled'' area, in
-fact interesting research questions are popping up as soon as one steps
-outside the classic automata theory (for example in terms of what kind
-of regular expressions are supported). The reason why it is
-interesting for us to look at the derivative approach introduced by
-Brzozowski for regular expression matching, and then much further
-developed by Sulzmann and Lu, is that derivatives can elegantly deal
-with some of the regular expressions that are of interest in ``real
-life''. This includes the not-regular expression, written $\neg\,r$
-(that is all strings that are not recognised by $r$), but also bounded
-regular expressions such as $r^{\{n\}}$ and $r^{\{n..m\}}$). There is
-also hope that the derivatives can provide another angle for how to
-deal more efficiently with back-references, which are one of the
-reasons why regular expression engines in JavaScript, Python and Java
-choose to not implement the classic automata approach of transforming
-regular expressions into NFAs and then DFAs---because we simply do not
-know how such back-references can be represented by DFAs.
-We also plan to implement the bitcoded algorithm
-in some imperative language like C to see if the inefficiency of the
-Scala implementation
-is language specific. To make this research more comprehensive we also plan
-to contrast our (faster) version of bitcoded algorithm with the
-Symbolic Regex Matcher, the RE2, the Rust Regex Engine, and the static
-analysis approach by implementing them in the same language and then compare
-their performance.
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