The set of strings that look like arithmetic expressions using nonnegative integers

and the operations $+,-,*,/,$ and parentheses to group terms.

For example, the following are properly formatted arithmetic expressions: $0,2,2+30,2+30*401,$

$(2+30)*401/(23+0),(((1+2)/3-4)*5+6)*7$

The following are not: $02,(2+30,2+30*401+,(2+30)*401),-4,2++3,(),2*(),((((1+2)*3-4)*5+6)*7$