I can use ANTLR to validate the syntax of an HTML document, but I also want to verify that its structure is valid. For example, I might want to require exactly one head and one body, allow one or more head items such as style, title, or script, and restrict body items to elements such as div, p, or h. These rules seem somewhat grammar-like, but they describe relationships and constraints on a parsed document rather than just the order of tokens. Should I express this with ANTLR, use a type or schema system, or parse the document first and validate the resulting object separately?
3 Answers
EBNF describes sequential syntax: what tokens can appear and in what order. It generally cannot express semantic conditions on an already-built representation. The usual solution is to parse the input into an AST or document tree, then run a separate validation pass over that structure.
ANTLR can support a two-stage design. Let the parser build a parse tree or AST, then use a visitor or listener to check structural and semantic rules while traversing it. You can also add semantic predicates for limited context-sensitive checks, but putting all validation into grammar actions usually makes the grammar harder to maintain.
You can model the finite part of this directly in a grammar. A rule such as html: declaration head body can enforce one head followed by one body, and nested rules can restrict which elements appear inside each section. Once the rules depend on symbol lookup, arbitrary relationships, or context outside the parse stack, use semantic validation code in addition to the grammar.
That makes sense for a fixed set of HTML elements. My concern is that constraints like “any number of bananas and grapes, but no tractors,” regardless of their order, feel more like set or object validation than a normal token sequence.
For a fixed HTML specification, an existing HTML parser and validator may be easier than defining a complete grammar yourself. A schema language can also work when the document model and constraints map cleanly to that schema.