See also [grammar_peg].

This module of [tcllib] 
can be used to create and manipulate Parsing Expression Grammars. 
These are similar but not equivalent to Context-Free Grammars 
(as should be familiar from [BNF] syntax specifications). 
Surprisingly, PEGs are both faster when matching (linear time 
as opposed to cubic time) and more powerful (at least no 
language has been shown context-free but not parsing 
expression) than CFGs.

An overview of Parsing Expression Grammars is given here [http://en.wikipedia.org/wiki/Parsing_expression_grammar] and the tcllib reference documentation is here [http://tcllib.sourceforge.net/doc/peg.html]

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