NPEG - .NET Parsing Expression Grammar

What is it?

• Implements PEG
  • Bryan Ford – PEG @ MIT 2004
  • Martin.Holzherr – 2008 codeproject
• Other Comparable Frameworks: LL(*) ANTLR , LALR(1) Irony
  • How do they handle predicates (lookahead/lookbehind) how far?
  • How they handle ambiguity
• Scannerless Parser (no need for scanner or parser code generation) – run in memory
• Infinite lookahead, doesn’t care about ambiguity – prioritized choice.
• Today: Can be deployed natively to C, C++, C#, and Java
  • Embedded, Mobile, Desktop
• TDD Built – so the framework has a solid regression harness. To continue it’s up keep.
• Can READ and WRITE itself! – DSL is written with NPEG.

Where is it used?

• Custom DSL (NPEG) – Domain Specific Languages
• Parse any proprietary format (Gerber files)
• Flexibleconfig
  • Configuration based on product, branch, developer, environment, task
    • Staging, production, production snapshot, disaster recovery environments
    • When web.config transformations become cumbersome
• SqlObfuscate
  • Production data back to development – RegexObfuscateStrategy
• Content Management System; widgets, markdown

Retail

How to start?

• Install-Package NPEG
• https://www.robusthaven.com/blog/parsing-expression-grammar/dsl-pair-programming-with-leblanc

Math Expression

cos(25)+cos(15+sin(25.333))+3

Hello world

string input = “hello world”;

            AExpression parseTree= PEGrammar.Load(@"(?< Expression >): 'Hello World'\i;");

visitor = new NpegParserVisitor(new StringInputIterator(input));

parseTree.Accept(visitor);

Assert.IsTrue(visitor.IsMatch);

AstNode node = visitor.AST;

Assert.IsTrue(node.Token.Name == "Expression");

Assert.IsTrue(node.Token.Value == input);
      
          

Backlog Items

• Language Workbench ***
  • Unit test terminals/nonterminals
  • Output to all supported languages
  • Better error reporting and debugging rules
• Save Memory - remove “Value” from the AstNode
  • Should be using iterator->Text(node.Start, node.End)
  • How they handle ambiguity
• Build a grammar optimizer
  • Collapse rules -> “x” “x” “x” -> “x”{3}
  • De Morgans Theorem
• Upcoming: Markdown
• Research:
  • Language Implementation

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