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Parsing Expression Grammars: a Recognition-Based Syntactic Foundation

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Parsing Expression Grammars: a Recognition-Based Syntactic Foundation Parsing Expression Grammars: A Recognition-Based Syntactic Foundation Bryan Ford Massachusetts Institute of Technology Cambridge, MA [email protected] Abstract 1 Introduction For decades we have been using Chomsky’s generative system of Most language syntax theory and practice is based on generative grammars, particularly context-free grammars (CFGs) and regu- systems, such as regular expressions and context-free grammars, in lar expressions (REs), to express the syntax of programming lan- which a language is defined formally by a set of rules applied re- guages and protocols. The power of generative grammars to ex- cursively to generate strings of the language. A recognition-based press ambiguity is crucial to their original purpose of modelling system, in contrast, defines a language in terms of rules or predi- natural languages, but this very power makes it unnecessarily diffi- cates that decide whether or not a given string is in the language. cult both to express and to parse machine-oriented languages using Simple languages can be expressed easily in either paradigm. For CFGs. Parsing Expression Grammars (PEGs) provide an alterna- example, fs 2 a∗ j s = (aa)ng is a generative definition of a trivial tive, recognition-based formal foundation for describing machine- language over a unary character set, whose strings are “constructed” oriented syntax, which solves the ambiguity problem by not intro- by concatenating pairs of a’s. In contrast, fs 2 a∗ j (jsj mod 2 = 0)g ducing ambiguity in the first place. Where CFGs express nondeter- is a recognition-based definition of the same language, in which a ministic choice between alternatives, PEGs instead use prioritized string of a’s is “accepted” if its length is even. choice. PEGs address frequently felt expressiveness limitations of CFGs and REs, simplifying syntax definitions and making it un- While most language theory adopts the generative paradigm, most necessary to separate their lexical and hierarchical components. A practical language applications in computer science involve the linear-time parser can be built for any PEG, avoiding both the com- recognition and structural decomposition, or parsing, of strings. plexity and fickleness of LR parsers and the inefficiency of gener- Bridging the gap from generative definitions to practical recogniz- alized CFG parsing. While PEGs provide a rich set of operators for ers is the purpose of our ever-expanding library of parsing algo- constructing grammars, they are reducible to two minimal recogni- rithms with diverse capabilities and trade-offs [9]. tion schemas developed around 1970, TS/TDPL and gTS/GTDPL, which are here proven equivalent in effective recognition power. Chomsky’s generative system of grammars, from which the ubiqui- tous context-free grammars (CFGs) and regular expressions (REs) arise, was originally designed as a formal tool for modelling and Categories and Subject Descriptors analyzing natural (human) languages. Due to their elegance and expressive power, computer scientists adopted generative grammars F.4.2 [Mathematical Logic and Formal Languages]: Gram- for describing machine-oriented languages as well. The ability of mars and Other Rewriting Systems—Grammar types; D.3.1 a CFG to express ambiguous syntax is an important and powerful [Programming Languages]: Formal Definitions and Theory— tool for natural languages. Unfortunately, this power gets in the Syntax; D.3.4 [Programming Languages]: Processors—Parsing way when we use CFGs for machine-oriented languages that are intended to be precise and unambiguous. Ambiguity in CFGs is General Terms difficult to avoid even when we want to, and it makes general CFG parsing an inherently super-linear-time problem [14, 23]. Languages, Algorithms, Design, Theory This paper develops an alternative, recognition-based formal foun- dation for language syntax, Parsing Expression Grammars or Keywords PEGs. PEGs are stylistically similar to CFGs with RE-like fea- tures added, much like Extended Backus-Naur Form (EBNF) no- Context-free grammars, regular expressions, parsing expression tation [30, 19]. A key difference is that in place of the unordered grammars, BNF, lexical analysis, unified grammars, scannerless choice operator ‘j’ used to indicate alternative expansions for a non- parsing, packrat parsing, syntactic predicates, TDPL, GTDPL terminal in EBNF, PEGs use a prioritized choice operator ‘=’. This operator lists alternative patterns to be tested in order, uncondition- ally using the first successful match. The EBNF rules ‘A ! a b j Permission to make digital or hard copies of all or part of this work for personal or a’ and ‘A ! a j a b’ are equivalent in a CFG, but the PEG rules ‘A classroom use is granted without fee provided that copies are not made or distributed a b = a’ and ‘A a = a b’ are different. The second alternative for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute in the latter PEG rule will never succeed because the first choice is to lists, requires prior specific permission and/or a fee. always taken if the input string to be recognized begins with ‘a’. POPL'04, January 14–16, 2004, Venice, Italy. Copyright 2004 ACM 1-58113-729-X/04/0001 ...$5.00 # Hierarchical syntax A PEG may be viewed as a formal description of a top-down parser. Grammar <- Spacing Definition+ EndOfFile Two closely related prior systems upon which this work is based Definition <- Identifier LEFTARROW Expression were developed primarily for the purpose of studying top-down parsers [4, 5]. PEGs have far more syntactic expressiveness than the Expression <- Sequence (SLASH Sequence)* LL(k) language class typically associated with top-down parsers, Sequence <- Prefix* however, and can express all deterministic LR(k) languages and Prefix <- (AND / NOT)? Suffix many others, including some non-context-free languages. Despite Suffix <- Primary (QUESTION / STAR / PLUS)? their considerable expressive power, all PEGs can be parsed in lin- Primary <- Identifier !LEFTARROW ear time using a tabular or memoizing parser [8]. These proper- / OPEN Expression CLOSE ties strongly suggest that CFGs and PEGs define incomparable lan- / Literal / Class / DOT guage classes, although a formal proof that there are context-free languages not expressible via PEGs appears surprisingly elusive. # Lexical syntax Identifier <- IdentStart IdentCont* Spacing Besides developing PEGs as a formal system, this paper presents IdentStart <- [a-zA-Z_] pragmatic examples that demonstrate their suitability for describ- IdentCont <- IdentStart / [0-9] ing realistic machine-oriented languages. Since these languages are generally designed to be unambiguous and linearly readable in the Literal <- [’] (![’] Char)* [’] Spacing first place, the recognition-oriented nature of PEGs creates a natural / ["] (!["] Char)* ["] Spacing affinity in terms of syntactic expressiveness and parsing efficiency. Class <- ’[’ (!’]’ Range)* ’]’ Spacing Range <- Char ’-’ Char / Char The primary contribution of this work is to provide language and Char <- ’\\’ [nrt’"\[\]\\] protocol designers with a new tool for describing syntax that is both / ’\\’ [0-2][0-7][0-7] practical and rigorously formalized. A secondary contribution is to / ’\\’ [0-7][0-7]? render this formalism more amenable to further analysis by prov- / !’\\’ . ing its equivalence to two simpler formal systems, originally named TS (“TMG recognition scheme”) and gTS (“generalized TS”) by LEFTARROW <- ’<-’ Spacing Alexander Birman [4, 5], in reference to an early syntax-directed SLASH <- ’/’ Spacing compiler-compiler. These systems were later called TDPL (“Top- AND <- ’&’ Spacing Down Parsing Language”) and GTDPL (“Generalized TDPL”) re- NOT <- ’!’ Spacing spectively by Aho and Ullman [3]. By extension we prove that with QUESTION <- ’?’ Spacing minor caveats TS/TDPL and gTS/GTDPL are equivalent in recog- STAR <- ’*’ Spacing nition power, an unexpected result contrary to prior conjectures [5]. PLUS <- ’+’ Spacing OPEN <- ’(’ Spacing The rest of this paper is organized as follows. Section 2 first defines CLOSE <- ’)’ Spacing PEGs informally and presents examples of their usefulness for de- DOT <- ’.’ Spacing scribing practical machine-oriented languages. Section 3 then de- fines PEGs formally and proves some of their important properties. Spacing <- (Space / Comment)* Section 4 presents useful transformations on PEGs and proves the Comment <- ’#’ (!EndOfLine .)* EndOfLine main result regarding the reducibility of PEGs to TDPL and GT- Space <- ’ ’ / ’\t’ / EndOfLine DPL. Section 5 outlines open problems for future study, Section 6 EndOfLine <- ’\r\n’ / ’\n’ / ’\r’ describes related work, and Section 7 concludes. EndOfFile <- !. Figure 1. PEG formally describing its own ASCII syntax 2 Parsing Expression Grammars The ?, *, and + operators behave as in common regular expression Figure 1 shows an example PEG, which precisely specifies a prac- syntax, except that they are “greedy” rather than nondeterministic. tical syntax for PEGs using the ASCII character set. The example The option expression ‘e?’ unconditionally “consumes” the text PEG describes its own complete syntax including all lexical char- matched by e if e succeeds, and the repetition expressions ‘e*’ acteristics. Most elements of the grammar should be immediately and ‘e+’ always consume as many successive matches of e as pos- recognizable to anyone familiar with CFGs and regular expressions. sible. The expression ‘a* a’ for
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