A simple lexical analyzer for CCstyle variable declarations

 // A simple lexical analyzer for C/C++-style variable declarations. // The grammar for the declarations is as follows: // // <declaration>  ::=     <type>  <var> ’;’ | <type> <var> ’=’ <number> ’;’ // <type>                 ::=     int | float // <var>          ::=     A | B | C | D | E // <number>       ::=     <integer> | <float> // <integer>      ::=     <integer> <digit> | <digit> // <digit>                ::=     0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 // <float>                ::=     <integer> ‘.’ <integer> //  // Write a lexical analyzer (or  // scanner) for the tokens used in the above grammar. The following // is the regular expression that defines the tokens: // //              = | ; | int | float | A | B | C | D | E | [0-9]+ | [0-9]+\\.[0-9]+ // // The program will read a declaration from the keyboard, and the // scanner you design should recognize and print out all tokens // included in the input. For example, given the following declaration: // //              int A = 123; // // your program should print: // // int // A // = // 123 // ; // // Make sure the program print out the token one per line in the order  // they appear in the input. Once an erroneous token is encountered, your // scanner should print out an error message and stopped scanning. For // example, given the following input: // //              int A = 0#; // // your program should print: // // int // A // = // 0 // #: Error: Unrecognizable token // // Note that tokens may NOT be separated by spaces. For example, the above // input: // //              int A = 123; // // does not have a space to separate 123 and ;. Also, the following inputs // are also legal and generate the same output: // //              int A=123; //              intA=123; //  // However, a whole token cannot be separated by spaces. For example, the // following input will cause 12 and 3 to be regarded as two distinct tokens. // //              int A = 12 3; // // The ouput will look like: // int // A // = // 12 // 3 // ; // // Also note that the scanner doesn\'t check for syntactic errors. Therefore the // above input is legal to this program. //  #include <iostream> #include <string>  using namespace std;  string GetToken(); void error(int);  int main() {         string token;          cout << \"Please enter a declaration in format \"                  << \"<type> <variable> [= number];\" << endl;          cout << \"The following are the tokens in the input:\" << endl;          token = GetToken();          while (token != \"\") {                 cout << token << endl;                 token = GetToken();         }          cout << \"Done!\" << endl;          return 0; }  string GetToken() {         string token;         char ch;          // Write the code here. Read the next token and store it in variable \"token\".         // The token must be read character by character. Use the regular expression         // defined above to extract tokens from the input.         //          // To read a character from keyboard, use:         //         //              cin.get(ch);         //         // where \"ch\" is a character variable.          return token; }