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# Rust grammar
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The Reference grammar is written in markdown code blocks using a modified BNF-like syntax (with a blend of regex and other arbitrary things). The [`mdbook-spec`] extension parses these rules and converts them to a renderable format, including railroad diagrams.
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The Reference grammar is written in markdown code blocks using a modified BNF-like syntax (with a blend of regex and other arbitrary things). The [`mdbook-spec`] extension parses these rules and converts them into a renderable format, including railroad diagrams.
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The code block should have a lang string with the word "grammar", a comma, and the category of the grammar, like this:
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The code block should have a lang string with the word `grammar`, a comma, and the category of the grammar, like this:
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~~~
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```grammar,items
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## Grammar syntax
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The syntax for the grammar itself is pretty close to what is described in the [Notation chapter], though there are some rendering differences.
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The syntax for the grammar itself is similar to what is described in the [Notation chapter], though there are some rendering differences.
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A "root" production, marked with `@root`, is one that is not used in any other production.
The general format is a series of productions separated by blank lines. The expressions are:
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The general format is a series of productions separated by blank lines. The expressions are as follows:
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| Expression | Example | Description |
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|------------|---------|-------------|
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| Unicode | U+0060 | A single unicode character. |
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| Unicode | U+0060 | A single Unicode character. |
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| NonTerminal | FunctionParameters | A reference to another production by name. |
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| Break ||This is used internally by the renderer to detect line breaks and indentation. |
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| Break ||Used internally by the renderer to detect line breaks and indentation. |
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| Comment | // Single line comment. | A comment extending to the end of the line. |
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| Terminal |\`example\`|This is a sequence of exact characters, surrounded by backticks |
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| Charset |[\`A\`-\`Z\`\`0\`-\`9\`\`_\`]| A choice from a set of characters, spaceseparated. There are three different forms. |
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| CharacterRange | [\`A\`-\`Z\`] | A range of characters, each character should be in backticks.
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| Terminal |\`example\`|A sequence of exact characters, surrounded by backticks.|
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| Charset |[\`A\`-\`Z\`\`0\`-\`9\`\`_\`]| A choice from a set of characters, space-separated. There are three different forms. |
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| CharacterRange | [\`A\`-\`Z\`] | A range of characters; each character should be in backticks.
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| CharacterTerminal |[\`x\`]| A single character, surrounded by backticks. |
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| CharacterName |[ LF ]| A nonterminal, referring to another production. |
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| Prose |\<any ASCII character except CR\>|This is an English description of what should be matched, surrounded in angle brackets. |
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| Group | (\`,\` Parameter)+ | This groups an expression for the purpose of precedence, such as applying a repetition operator to a sequence of other expressions.
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| Prose |\<any ASCII character except CR\>|An English description of what should be matched, surrounded in angle brackets. |
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| Group | (\`,\` Parameter)+ | Groups an expression for the purpose of precedence, such as applying a repetition operator to a sequence of other expressions.
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| NegativeExpression |~[\`\` LF]| Matches anything except the given Charset, Terminal, or Nonterminal. |
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| Sequence |\`fn\` Name Parameters | A sequence of expressions, where they must match in order. |
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| Sequence |\`fn\` Name Parameters | A sequence of expressions that must match in order. |
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| Alternation | Expr1 \| Expr2 | Matches only one of the given expressions, separated by the vertical pipe character. |
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| Suffix |\_except \[LazyBooleanExpression\]\_|This adds a suffix to the previous expression to provide an additional English description to it, rendered in subscript. This can have limited markdown, but try to avoid anything except basics like links. |
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| Footnote |\[^extern-safe\]|This adds a footnote, which can supply some extra information that may be helpful to the user. The footnote itself should be defined outside of the code block like a normal markdown footnote. |
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| Suffix |\_except \[LazyBooleanExpression\]\_|Adds a suffix to the previous expression to provide an additional English description, rendered in subscript. This can contain limited markdown, but try to avoid anything except basics like links. |
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| Footnote |\[^extern-safe\]|Adds a footnote, which can supply extra information that may be helpful to the user. The footnote itself should be defined outside of the code block like a normal markdown footnote. |
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| Optional | Expr? | The preceding expression is optional. |
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| Repeat | Expr*| The preceding expression is repeated 0 or more times. |
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| Repeat (non-greedy) | Expr*? | The preceding expression is repeated 0 or more times without being greedy. |
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| RepeatPlus | Expr+ | The preceding expression is repeated 1 or more times. |
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| RepeatPlus (non-greedy) | Expr+? | The preceding expression is repeated 1 or more times without being greedy. |
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| RepeatRange | Expr{2..4} | The preceding expression is repeated between the range of times specified. Either bounds can be excluded, which works just like Rust ranges. |
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| RepeatRange | Expr{2..4} | The preceding expression is repeated between the range of times specified. Either bound can be excluded, which works just like Rust ranges. |
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## Automatic linking
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The [`mdbook-spec`] plugin automatically adds markdown link definitions for all the production names on every page. If you want to link directly to a production name, all you need to do is surround it in square brackets, like `[ArrayExpression]`.
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The [`mdbook-spec`] plugin automatically adds markdown link definitions for all production names on every page. To link directly to a production name, simply surround it in square brackets, like `[ArrayExpression]`.
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In some cases there might be name collisions with the automatic linking of rule names. In that case, disambiguate with the `grammar-` prefix, such as `[Type][grammar-Type]`. You can also do that if you just feel like being more explicit.
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In some cases, there might be name collisions with the automatic linking of rule names. In that case, disambiguate with the `grammar-` prefix, such as `[Type][grammar-Type]`. You can also do this if you prefer to be more explicit.
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