Scannerless parsing

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.

Advantages

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) ^[1]

Disadvantages

Since the lexical scanning and syntactic parsing are combined, the resulting parser tends to be harder to understand and debug for more-complex languages

Required extensions

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

Implementations

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.

Notes

^ 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.