examples/

README

EBNF Grammar Examples

This directory contains example grammars demonstrating various features of the EBNF parser.

Getting Started

arithmetic.ebnf

Difficulty: Beginner

A simple arithmetic expression grammar showing:

• Basic EBNF structure
• Operator precedence through grammar hierarchy (expr → term → factor)
• Hidden tokens for parentheses and whitespace
• Regex patterns for numbers

Perfect for understanding the basics.

json.ebnf

Difficulty: Intermediate

A complete JSON grammar based on the official specification at https://www.json.org/

Demonstrates:

• Recursive structures (objects and arrays containing values)
• Character-by-character string parsing with escape sequences
• Optional parts (?) for number fractions and exponents
• Hidden structural tokens (braces, brackets, quotes)
• Complex choice expressions

This is a real-world example showing how to parse a well-known format.

regex_demo.ebnf

Difficulty: Intermediate

Shows modern regex pattern features:

• #"regex" syntax for complex patterns
• Practical patterns (identifiers, emails, URLs)
• Comparison between character-by-character rules vs regex

Good for learning when to use regex patterns vs traditional EBNF.

instaparse_demo.ebnf

Difficulty: Advanced

Demonstrates PEG (Parsing Expression Grammar) style features:

• Ordered choice (/) where first match wins
• Hidden expressions (<>) to simplify AST
• Expression language with proper precedence

Shows how to write grammars that are deterministic and unambiguous.

Running the Examples

Parsing with a Grammar

package main

import (
    "fmt"
    "log"
    "os"
    "github.com/wbrown/ebnf"
    "github.com/wbrown/ebnf/parse"
)

func main() {
    // Load the arithmetic grammar
    grammar, err := ebnf.LoadGrammar("examples/arithmetic.ebnf")
    if err != nil {
        log.Fatal(err)
    }

    // Create a parser
    parser := parse.New(grammar)

    // Parse an arithmetic expression
    input := "2 + 3 * 4"
    tree, err := parser.Parse(input, "expr")
    if err != nil {
        log.Fatal(err)
    }

    // Print the parse tree
    parse.PrintAST(os.Stdout, tree)
}

Using the CLI Tool

A command-line tool is provided for quick testing:

# Build the tool
go build -o ebnf-parse ./cmd/ebnf-parse

# Parse JSON
echo '{"name": "Alice"}' | ./ebnf-parse -grammar examples/json.ebnf

# Parse arithmetic with compact output
echo "2 + 3 * 4" | ./ebnf-parse -grammar examples/arithmetic.ebnf -rule expr -compact

# Enable debug output to see parsing trace
./ebnf-parse -grammar examples/json.ebnf -input data.json -debug

See examples_test.go and parse/*_test.go for more examples of loading and working with these grammars.