What is Pegex?

The name "Pegex" comes from PEG + Regex. With Pegex you define top down grammars that eventually break down to regex fragments. ie The low level parsing matches are always done with regexes against the current position in the input stream.

What is Parsing?

Usually a parser gets its instructions of what means what from something called a grammar. A grammar is a set of rules that defines how the input must be structured. In many parsing methodologies, input is preprocessed (possibly into tokens) before the parser/grammar get to look at it. Although this is a common method, it is not the only approach.

How Pegex Works

Parser

The Pegex parsing engine

Grammar

The rules of a particular syntax

Receiver

The logic for processing matches

Input

Text conforming to the grammar rules

Quite simply, a parser object is created with a grammar object and a receiver object. Then the parser object's "parse()" method is called on an input object. The parser applies the rules of the grammar to the input and invokes methods of the receiver as the rules match. The parse is either successful or results in an error. The result is whatever the receiver object decides it should be.

For example consider a parser that turns the Markdown text language into HTML. The Pegex code to use this might look like this:

In the simplest terms, Pegex works like this (pseudocode):

    parser = new Pegex.Parser(
        grammar: new Markdown.Grammar
        receiver: new Markdown.Receiver.HTML
    )
    html = parser.parse(markdown)

See Also

• Pegex::API
• Pegex::Syntax
• Pegex::Tutorial