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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