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\title{{POSIX} {L}exing with {B}itcoded {D}erivatives}

\titlerunning{POSIX Lexing with Bitcoded Derivatives}

\author{Chengsong Tan\inst{1,2} \and Christian Urban\inst{2}}

\institute{Imperial College London \and King's College London\\

\email{\{chengsong.tan,christian.urban\}@kcl.ac.uk}}

%!\author{Chengsong Tan}{King's College London}{chengsong.tan@kcl.ac.uk}{}{}

%!\author{Christian Urban}{King's College London}{christian.urban@kcl.ac.uk}{}{}

%!\authorrunning{C.~Tan and C.~Urban}

%!\keywords{POSIX matching and lexing, derivatives of regular expressions, Isabelle/HOL}

%!\category{}

%!\ccsdesc[100]{Design and analysis of algorithms}

%!\ccsdesc[100]{Formal languages and automata theory}

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\begin{document}

\maketitle

\begin{abstract}

  Sulzmann and Lu describe a lexing algorithm that calculates

  Brzozowski derivatives using bitcodes annotated to regular

  expressions.  Their algorithm generates POSIX values which encode

  the information of \emph{how} a regular expression matches a

  string---that is, which part of the string is matched by which part

  of the regular expression. This information is needed in the

  context of lexing in order to extract and to classify tokens.

  The purpose of the bitcodes is to generate POSIX values incrementally while

  derivatives are calculated. They also help with designing

  an ``aggressive'' simplification function that keeps the size of

  derivatives finitely bounded. Without simplification the size of some derivatives can grow

  arbitrarily big, resulting in an extremely slow lexing algorithm.  In this

  paper we describe a variant of Sulzmann and Lu's algorithm: Our

  variant is a recursive functional program, whereas Sulzmann

  and Lu's version involves a fixpoint construction. We \textit{(i)}

  prove in Isabelle/HOL that our variant is correct and generates

  unique POSIX values (no such proof has been given for the

  original algorithm by Sulzmann and Lu); we also \textit{(ii)} establish finite

  bounds for the size of our derivatives.

  %The size can be seen as a

  %proxy measure for the efficiency of the lexing algorithm: because of

  %the polynomial bound our algorithm does not suffer from

  %the exponential blowup in earlier works.

  % Brzozowski introduced the notion of derivatives for regular

  % expressions. They can be used for a very simple regular expression

  % matching algorithm.  Sulzmann and Lu cleverly extended this

  % algorithm in order to deal with POSIX matching, which is the

  % underlying disambiguation strategy for regular expressions needed

  % in lexers.  Their algorithm generates POSIX values which encode

  % the information of \emph{how} a regular expression matches a

  % string---that is, which part of the string is matched by which

  % part of the regular expression.  In this paper we give our

  % inductive definition of what a POSIX value is and show $(i)$ that

  % such a value is unique (for given regular expression and string

  % being matched) and $(ii)$ that Sulzmann and Lu's algorithm always

  % generates such a value (provided that the regular expression

  % matches the string). We show that $(iii)$ our inductive definition

  % of a POSIX value is equivalent to an alternative definition by

  % Okui and Suzuki which identifies POSIX values as least elements

  % according to an ordering of values.  We also prove the correctness

  % of Sulzmann's bitcoded version of the POSIX matching algorithm and

  % extend the results to additional constructors for regular

  % expressions.  \smallskip

\end{abstract}

\input{session}

\end{document}

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