代写一个字符串表达式解析器引擎，并计算表达式的值。RequirementYour task is to write a program that will read expressions consisting of strings and operations from a file, and evaluate and show the output of the given expression.String expressions consist of strings, the operators + (concatenate), - (remove), < (remove end), and > (remove start), along with parentheses to specify a desired order of operations. The < operator indicates you should delete the last character of the string, unless the string consists of only a single character, in which case you should not change the string. The > operator indicates you should delete the first character of the string, unless the string consists of only a single character, in which case you should not change the string. The + operator indicates you should append the second string to the end of the first string. The - operator indicates that you should remove the first instance of the second string from the first string (the first string is returned unmodified if there is no match).String expressions are defined formally as follows:Any string of 1 or more letters a-z (lower-case only) is a string expression.If Y1, Y2, …, Yk are string expressions (for k > 1) then the following are string expressions:

Y1(Y1-Y2)(Y1+Y2+Y3+...+Yk)Notice that our format rules out the expression ab+bc, since it is missing the parentheses. It also rules out (ab+bc-b) which would have to instead be written ((ab+bc)-b), so you never have to worry about precedence. This should make your parsing task significantly easier. Whitespace may occur in arbitrary places in string expressions, but never in the middle of a string of letters. Each expression will be on a single line.Examples (the first four are valid, the other four are not):(<>yoda+go )) // evaluates to b <> <<<((eve + boo+buzz) - >< ((<>zelda)- ><