Mermaid.js Railroad EBNF Syntax Guide

Mermaid.js provides dedicated syntax designed specifically to build Railroad diagrams from Extended Backus-Naur Form (EBNF) definitions. With plain-text production rules, you can visually represent formal language grammars, parser expressions, and command syntaxes using intuitive visual tracks and decision branches.

1. Core EBNF Operators & Syntax

In EBNF syntax diagrams, production rules use standard formal grammar notation to define terminals, non-terminals, choices, and repetitions along a visual track.

EBNF Notation	Railroad Meaning	Description
`rule = ... ;`	Production Rule	Defines a named grammar rule ended with a semicolon
`"literal"`	Terminal Node	Represents exact characters or keywords enclosed in double quotes
`\|`	Choice Branch	Branches the track into mutually exclusive alternatives
`( ... )*`	Zero-or-More Loop	Creates a returning loop track for elements that can repeat 0 or more times
`+`	One-or-More Loop	Indicates an element that must occur at least once

Mermaid Edit Mermaid in VPasCode

railroad-ebnf-beta
    title "Basic Token Rule"

    identifier = letter ( letter | digit )* ;
    letter = "a" | "b" | "c" ;
    digit = "0" | "1" | "2" ;

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2. Arithmetic Expression Grammar Example

Use production rules to model nested language structures such as mathematical expressions, operator precedence, and parenthesized grouping.

Mermaid Edit Mermaid in VPasCode

railroad-ebnf-beta
    title "Arithmetic Expression Grammar"

    expression = term ( ( "+" | "-" ) term )* ;
    term = factor ( ( "*" | "/" ) factor )* ;
    factor = number | "(" expression ")" ;
    number = digit+ ;
    digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;

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3. Command & Query Language Example

The following example demonstrates a complete SQL query production rule set mapped directly from EBNF notation into a visual railroad syntax diagram.

Mermaid Edit Mermaid in VPasCode

railroad-ebnf-beta
    title "SQL Statement Grammar"

    select_statement = "SELECT" column_clause "FROM" table_name [ where_clause ] ;
    column_clause = "*" | column_name ( "," column_name )* ;
    where_clause = "WHERE" condition ;
    table_name = identifier ;
    column_name = identifier ;
    identifier = "a" | "b" | "c" ;

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Conclusion

Using Mermaid’s EBNF syntax gives language designers, compiler engineers, and technical writers a clean, code-driven way to document formal grammars. By declaring production rules directly, you can automatically generate self-documenting syntax diagrams alongside your technical specifications.