Pegasus

Pegasus is a PEG-style parser generator for C# that integrates with MSBuild and Visual Studio.

Visual Studio Extension

Getting Started

The easiest way to get a copy of Pegasus is to install the Pegasus NuGet package in Visual Studio.

PM> Install-Package Pegasus

Due to a limitation in Visual Studio 2017 and earlier, you will need to reload your project for the 'PegGrammar' build action to be recognized.

Once you have the package installed, files in your project marked as 'PegGrammar' in the properties window will be compiled to their respective .peg.g.cs parser classes before every build. These parser classes will be automatically included in compilation.

For help with grammar syntax, see the syntax guide wiki entry

Example

Here is an example of a simple parser for mathematical expressions:

@namespace PegExamples
@classname ExpressionParser
@using System.Globalization

additive <double> -memoize
    = left:additive "+" right:multiplicative { left + right }
    / left:additive "-" right:multiplicative { left - right }
    / multiplicative

multiplicative <double> -memoize
    = left:multiplicative "*" right:power { left * right }
    / left:multiplicative "/" right:power { left / right }
    / power

power <double>
    = left:primary "^" right:power { Math.Pow(left, right) }
    / primary

primary <double> -memoize
    = decimal
    / "-" primary:primary { -primary }
    / "(" additive:additive ")" { additive }

decimal <double>
    = value:([0-9]+ ("." [0-9]+)?) { double.Parse(value, CultureInfo.InvariantCulture) }

This will take mathematical expressions as strings and evaluate them with the proper order of operations and associativity to produce a result as a decimal.

The above parser would be used like so:

var parser = new PegExamples.ExpressionParser();
var result = parser.Parse("5.1+2*3");
Console.WriteLine(result); // Outputs "11.1".