In computer science, scannerless parsing (also called lexerless parsing) performs tokenization (breaking a stream of characters into words) and parsing (arranging the words into phrases) in a single step, rather than breaking it up into a pipeline of a lexer followed by a parser, executing concurrently. A language grammar is scannerless if it uses a single formalism to express both the lexical (word level) and phrase level structure of the language.
Dividing processing into a lexer followed by a parser is more modular; scannerless parsing is primarily used when a clear lexer–parser distinction is unneeded or unwanted. Examples of when this is appropriate include TeX, most wiki grammars, makefiles, simple application-specific scripting languages, and Raku.
- Only one metalanguage is needed
- Non-regular lexical structure is handled easily
- "Token classification" is unneeded which removes the need for design accommodations such as "the lexer hack" and language reserved words (such as "while" in C)
- Grammars can be compositional (can be merged without human intervention)
- Since the lexical scanning and syntactic parsing are combined, the resulting parser tends to be harder to understand and debug for more-complex languages
Eelco Visser identified five key extensions to classical context-free syntax which handle almost all common non-context-free constructs arising in practice:
- Follow restrictions, a limited form of "longest match"
- Reject productions, a limited form of negative matching (as found in boolean grammars)
- Preference attributes to handle the dangling else construct in C-like languages
- Per-production transitions rather than per-nonterminal transitions in order to facilitate:
- Associativity attributes, which prevent a self-reference in a particular production of a nonterminal from producing that same production
- Precedence/priority rules, which prevent self-references in higher-precedence productions from producing lower-precedence productions
- SGLR is a parser for the modular Syntax Definition Formalism SDF, and is part of the ASF+SDF Meta-Environment and the Stratego/XT program transformation system.
- JSGLR, a pure Java implementation of SGLR, also based on SDF.
- TXL supports character-level parsing.
- dparser generates ANSI C code for scannerless GLR parsers.
- Spirit allows for both scannerless and scanner-based parsing.
- SBP is a scannerless parser for boolean grammars (a superset of context-free grammars), written in Java.
- Laja is a two phase scannerless parser generator with support for mapping the grammar rules into objects, written in Java.
- The Raku Grammars feature of the general purpose programming language Raku.
- PyParsing is a scannerless parser written in pure Python.
- META II Has built in token parsers functions.
- TREE-META Like META II also is scannerless having builtin lexer functions.
- CWIC Compiler for Writing and Implementing Compilers. Has token rules as part of its language. Rules in CWIC were compiled into boolean functions returning success or failure.
- UniCC is a LALR parser generator for different programming languages supporting optional scannerless parser construction by automatic grammar revision regarding character-classes.
- ^ This is because parsing at the character level makes the language recognized by the parser a single context-free language defined on characters, as opposed to a context-free language of sequences of strings in regular languages. Some lexerless parsers handle the entire class of context-free languages, which is closed under composition.
- Visser, E. (August 1997). Scannerless Generalized-LR Parsing. The Netherlands: University of Amsterdam. Retrieved 22 November 2012.