SDF is a formalism for the definition of syntax which is comparable to BNF in some respects, but h_ as a wider scope in that it also covers the definition of lexical and abstract syntax. Its design and implementation are tailored towards the language designer who wants to develop new languages as well as implement existing ones in a highly interactive manner. It emphasizes comP,actness of syntax definitions by offering (a) a standard interface between lexical and context-free syntax; (b) a standard correspondence between context-free and abstract syntax; (c) pow~rful disambiguation and list constructs; and (d) an efficient incremental implementation which accepts arbitrary context-free syntax definitions. SDF can be combined with a variety of progr:amming and specification languages. In this way these obtain fully general user-definable syntax.

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An LR-based parser generator for arbitrary context-free grammars is described, which generates parsers by need and processes grammar modifications by updating already existing parsers. We motivate the need for these techniques in the context of interactive language definition environments, present all required algorithms, and give measurements comparing their performance with that of conventional techniques.

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It is common practice to specify textual patterns by means of a set of regular expressions and to transform this set into a finite automaton to be used for the scanning of input strings. In many applications, the cost of this preprocessing phase can be amortized over many uses of the constructed automaton. In this paper new techniques for lazy and incremental scanner generation are presented. The lazy technique postpones the construction of parts of the automaton until they are really needed during the scanning of input. The incremental technique allows modifications to the original set of regular expressions to be made and reuses major parts of the previous automaton. This is interesting in applications such as environments for the interactive development of language definitions in which modifications to the definition of lexical syntax and the uses of the generated scanners alternate frequently.

A su bstring recognizer for a language L determines whether a string s is a substring of a sentence in L, i.e., substring-recognize(s) succeeds if and only if 3v , w: vsw E L. The algorithm for sub string recognition presented here accepts general context-free grammars and uses the same parse tables as the parsing algorithm from which it was derived. Substring recognition is useful for non correcting syntax error recovery and for incremen tal parsing. By extending the substring recognizer with the ability to generate trees for the possible contextual completions of the substring, we obtain a substring parser, which can be used in a syntax directed editor to complete fragments of sentences.