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Parsing / Railroad Diagrams

Railroad Diagrams (or Syntax Diagrams) are the graphical way to represent context-free grammars.

Each diagram defines a nonterminal. And they must have an entry point and an endpoint.

There is no official standard about how should we draw the railroad/syntax diagrams but historically, terminals are represented by round boxes and nonterminals are represented by rectangular boxes.

Let’s take a look at the following grammar:

sum = signed digit, [ "+", signed digit ] ;
signed digit = [ "-" ], digit ;
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;

The railroad diagram of the above grammar can be represented as:

digit

signed digit

sum

In real world, syntax diagram can be seen in JSON specs^◹, or SQLite^◹.