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In computer science, a parsing expression grammar, or PEG, is a type of analytic formal grammar, i.e. it describes a formal language in terms of a set of rules for recognizing strings in the language. The formalism was introduced by Bryan Ford in 2004 and is closely related to the family of top-down parsing languages introduced in the early 1970s. Read more on Wikipedia...
Expr ← Sum Sum ← Product (('+' / '-') Product)* Product ← Value (('*' / '/') Value)* Value ← [0-9]+ / '(' Expr ')'
Value ← [0-9.]+ / '(' Expr ')' Product ← Expr (('*' / '/') Expr)* Sum ← Expr (('+' / '-') Expr)* Expr ← Product / Sum / Value