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Recursive descent LL(k) parser for .NET with Fluent API, BNF, EBNF and Gold Grammars
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A recursive descent LL(k) parser framework for .NET



Eto.Parse is a highly optimized recursive decent parser framework that can be used to create parsers for context-free grammars that go beyond the capability of regular expressions.

You can use BNF, EBNF, or Gold Meta-Language grammars to define your parser, code them directly using a Fluent API, or use shorthand operators (or a mix of each).

Why not use RegEx?

Regular Expressions work great when the syntax is not complex, but fall short especially when dealing with any recursive syntax using some form of brackets or grouping concepts.

For example, creating a math parser using RegEx cannot validate (directly) that there are matching brackets. E.g. "((1+2)*3)", or "{ 'my': 'value', 'is' : {'recursive': true } }"


The framework has been put together to get at the relevant values as easily as possible. Each parser can be named, which then builds a tree of named matches that represent the interesting sections of the parsed input. You can use events on the named sections to perform logic when they match, or just parse the match tree directly.

Left Recursion

One rather cumbersome issue to deal with using recursive descent parsers is left recursion. Eto.Parse automatically identifies left recursive grammars and transforms them into a repeating pattern.


Eto.Parse has been highly optimized for performance and memory usage. For example, here's a comparison parsing a large JSON string 1000 times (times in seconds):


Test Parsing Slower than best Warmup Slower than best
Eto.Parse 2,327s 1,00x 0,008s 1,00x
Newtonsoft Json 2,523s 1,08x 0,068s 8,08x
ServiceStack.Text 2,854s 1,23x 0,066s 7,78x
Irony 25,401s 10,92x 0,188s 22,28x
bsn.GoldParser 11,186s 4,81x 0,013s 1,49x
NFX.JSON 11,847s 5,09x 0,187s 22,10x
SpracheJSON 92,774s 39,88x 0,189s 22,37x

(Warmup is the time it takes to initialize the engine for the first time and perform the first parse of the json string).

Memory & Objects

Framework Allocated More than best # Objects More than best
Eto.Parse 553.99 MB 1.00x 15268050 1.00x
Newtonsoft.Json 1,074.27 MB 1.94x 21562432 1.41x
ServiceStack.Text 2,540.91 MB 4.59x 15738493 1.03x
Irony 4,351.44 MB 7.85x 94831118 6.21x
bsn.GoldParser 2,012.16 MB 3.63x 74387176 4.87x


For example, the following defines a simple hello world parser in Fluent API:

// optional repeating whitespace
var ws = Terminals.WhiteSpace.Repeat(0);

// parse a value with or without brackets
var valueParser = Terminals.Set('(')

// our grammar
var grammar = new Grammar(

Or using shorthand operators:

// optional repeating whitespace
var ws = -Terminals.WhiteSpace;

// parse a value with or without brackets
Parser valueParser = 
	('(' & ws & (+Terminals.AnyChar ^ (ws & ')')).Named("value") & ws & ')')
	| (+!Terminals.WhiteSpace).Named("value");

// our grammar
var grammar = new Grammar(
	ws & valueParser.Named("first") & 
	ws & valueParser.Named("second") & 
	ws & Terminals.End

Or, using EBNF:

var grammar = new EbnfGrammar().Build(@"
(* := is an extension to define a literal with no whitespace between repeats and sequences *)
ws := {? Terminals.WhiteSpace ?};

letter or digit := ? Terminals.LetterOrDigit ?;

simple value := letter or digit, {letter or digit};

bracket value = simple value, {simple value};

optional bracket = '(', bracket value, ')' | simple value;

first = optional bracket;

second = optional bracket;

grammar = ws, first, second, ws;
", "grammar");

These can parse the following text input:

var input = "  hello ( parsing world )  ";
var match = grammar.Match(input);

var firstValue = match["first"]["value"].Value;
var secondValue = match["second"]["value"].Value;

firstValue will equal "hello", and secondValue will equal "parsing world".


Licensed under MIT.

See LICENSE file for full license.

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