Optimally Streaming Greedy Regular Expression Parsing

Abstract. We study the problem of streaming regular expression parsing: Given a regular expression and an input stream of symbols, how to output a serialized syntax tree representation as an output stream during input stream processing. We show that optimally streaming regular expression parsing, outputting bits of the output as early as is semantically possible for any regular expression of size m and any input string of length n, can be performed in time O(2 m log m + mn) on a unit-cost random-access machine. This is for the wide-spread greedy disambiguation strategy for choosing parse trees of grammatically ambiguous regular expressions. In particular, for a fixed regular expression, the algorithm's run-time scales linearly with the input string length. The exponential is due to the need for preprocessing the regular expression to analyze state coverage of its associated NFA, a PSPACE-hard problem, and tabulating all reachable ordered sets of NFA-states. Previous regular expression parsing algorithms operate in multiple phases, always requiring processing or storing the whole input string before outputting the first bit of output, not only for those regular expressions and input prefixes where reading to the end of the input is strictly necessary

Anagopos: A Reduction Graph Visualizer for Term Rewriting and Lambda Calculus

We present Anagopos, an open source tool for visualizing reduction graphs of terms in lambda calculus and term rewriting. Anagopos allows step-by-step generation of reduction graphs under six different graph drawing algorithms. We provide ample examples of graphs drawn with the tool

A Typed, Algebraic Approach to Parsing

In this paper, we recall the definition of the context-free expressions (or µ-regular expressions), an algebraic presentation of the context-free languages. Then, we define a core type system for the context-free expressions which gives a compositional criterion for identifying those context-free expressions which can be parsed unambiguously by predictive algorithms in the style of recursive descent or LL(1). Next, we show how these typed grammar expressions can be used to derive a parser combinator library which both guarantees linear-time parsing with no backtracking and single-token lookahead, and which respects the natural denotational semantics of context-free expressions. Finally, we show how to exploit the type information to write a staged version of this library, which produces dramatic increases in performance, even outperforming code generated by the standard parser generator tool ocamlyacc

Infinitary axiomatization of the equational theory of context-free languages

We give a natural complete infinitary axiomatization of the equational theory of the context-free languages, answering a question of Leiß (1992)

Infinitary Axiomatization of the Equational Theory of Context-Free Languages

We give a natural complete infinitary axiomatization of the equational theory of the context-free languages, answering a question of Leiß (1992)