Formal grammar is strictly defined as a quadruple G=(V, T, P, S), where V and T are finite argument sets and terminators respectively, and V and T have no common elements, that is, V ∩ T =&; AElig。 S is a special parameter called the start symbol. P is a finite set of generating formulas. The basic form of the generative formula is: a→β, where both a and β are elements in (V∪T)*, that is, they are both symbol strings composed of independent variables and terminators, but A is required to contain at least one non-terminator. In the definition of formal grammar, generating set P is very important. After making some conventions on the use of symbols, we can infer the arguments, terminators and starting symbols of a grammar only by checking the generated expressions, so we can define a formal grammar by listing the generated expressions.