This example is based on the Wikipedia entry on Parsing Expression Grammar. The parser calculates intermediate expressions and applies operations returning the value of the expression.
The Grammar is expressed as follows:
[1] Expr ::= Sum
[2] Sum ::= Product (('+' | '-') Product)*
[3] Product ::= Power (('*' | '/') Power)*
[4] Power ::= Value ('^' Power)?
[5] Value ::= NUMBER | '(' Expr ')'
[6] NUMBER ::= [0-9]+
The calculator is expressed in calc.rb and can be exercised using the calc wrapper, but at its simplest can be invoked as follows:
require 'calc'
calc = Calc.new
result = calc.evaluate('1 + 2 * 3')
#=> 7