问题：编写Lex程序以识别有效的算术表达式并识别标识符和运算符。
解释：
Flex(快速词法分析器生成器)是由Vern Paxson在1987年左右用C编写的用于生成词法分析器(扫描器或词法分析器)的工具/计算机程序。Lex读取指定词法分析器的输入流，并输出在C语言中实现词法分析器的源代码编程语言。函数yylex()是运行规则部分的主要flex函数。
例子：
Input: a+b*c
Output: valid expression
the operators are :
+
*
the identifiers are :
a
b
c
Input: a+b-
Output: invalid expression
Input: (a*b)
Output: valid expression
the operators are :
*
the identifiers are :
a
b
Input: (a+b-
Output: invalid expression 执行：
/* Lex program to recognize valid arithmetic expression
and identify the identifiers and operators */
%{
#include
#include
int operators_count = 0, operands_count = 0, valid = 1, top = -1, l = 0, j = 0;
char operands[10][10], operators[10][10], stack[100];
%}
%%
"(" {
top++;
stack[top] = '(';
}
"{" {
top++;
stack[top] = '{';
}
"[" {
top++;
stack[top] = '[';
}
")" {
if (stack[top] != '(') {
valid = 0;
}
else if(operands_count>0 && (operands_count-operators_count)!=1){
valid=0;
}
else{
top--;
operands_count=1;
operators_count=0;
}
}
"}" {
if (stack[top] != '{') {
valid = 0;
}
else if(operands_count>0 && (operands_count-operators_count)!=1){
valid=0;
}
else{
top--;
operands_count=1;
operators_count=0;
}
}
"]" {
if (stack[top] != '[') {
valid = 0;
}
else if(operands_count>0 && (operands_count-operators_count)!=1){
valid=0;
}
else{
top--;
operands_count=1;
operators_count=0;
}
}
"+"|"-"|"*"|"/" {
operators_count++;
strcpy(operators[l], yytext);
l++;
}
[0-9]+|[a-zA-Z][a-zA-Z0-9_]* {
operands_count++;
strcpy(operands[j], yytext);
j++;
}
%%
int yywrap()
{
return 1;
}
int main()
{
int k;
printf("Enter the arithmetic expression: ");
yylex();
if (valid == 1 && top == -1) {
printf("\nValid Expression\n");
}
else
printf("\nInvalid Expression\n");
return 0;
}
输出：