# Lex program to recognize valid arithmetic expression and identify the identifiers and operators

Problem: Write a Lex program to recognize valid arithmetic expression and identify the identifiers and operators.

Explanation:
Flex (Fast lexical Analyzer Generator) is a tool/computer program for generating lexical analyzers (scanners or lexers) written by Vern Paxson in C around 1987. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language. The function yylex() is the main flex function which runs the Rule Section.

Examples:

```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 ```

Implementation:

 `/* Lex program to recognize valid arithmetic expression ` `        ``and identify the identifiers and operators */` `% ` `{ ` `/* Definition section */` `#include ` `#include ` `    ``int` `noprt = 0, nopnd = 0, valid = 1, top = -1, l = 0, j = 0; ` `    ``char` `opnd, oprt, a; ` `    ``% ` `} ` ` `  `/* Rule Section */` `% % ``"("` `{ ` `    ``top++; ` `    ``a[top] = ``'('``; ` `} ` `"{"` `{ ` `    ``top++; ` `    ``a[top] = ``'{'``; ` `} ` `"["` `{ ` `    ``top++; ` `    ``a[top] = ``'['``; ` `} ` `")"` `{ ` `    ``if` `(a[top] != ``'('``) { ` `        ``valid = 0; ` `        ``return``; ` `    ``} ` `    ``else` `        ``top--; ` `} ` `"}"` `{ ` `    ``if` `(a[top] != ``'{'``) { ` `        ``valid = 0; ` `        ``return``; ` `    ``} ` `    ``else` `        ``top--; ` `} ` `"]"` `{ ` `    ``if` `(a[top] != ``'['``) { ` `        ``valid = 0; ` `        ``return``; ` `    ``} ` `    ``else` `        ``top--; ` `} ` `"+"` `| ``"-"` `| ``"*"` `| ``"/"` `{ ` `    ``noprt++; ` `    ``strcpy``(oprt[l], yytext); ` `    ``l++; ` `} ` `[0 - 9] + | [a - zA - Z][a - zA - Z0 - 9_]* ` `{ ` `    ``nopnd++; ` `    ``strcpy``(opnd[j], yytext); ` `    ``j++; ` `} ` `% % ` ` `  `    ``// driver code ` `    ``main() ` `{ ` `    ``int` `k; ` `    ``printf``(``"Enter the arithmetic expression: "``); ` `    ``yylex(); ` ` `  `    ``if` `(valid == 1 && top == -1 && (nopnd - noprt) == 1) { ` `        ``printf``(``"valid expression\n"``); ` ` `  `        ``printf``(``"the operators are :\n"``); ` `        ``for` `(k = 0; k < l; k++) ` `            ``printf``(``"%s \n"``, oprt[k]); ` ` `  `        ``printf``(``"the identifiers are :\n"``); ` `        ``for` `(k = 0; k < j; k++) ` `            ``printf``(``"%s \n"``, opnd[k]); ` `    ``} ` `    ``else` `        ``printf``(``"invalid expression\n"``); ` `} ` ` `  `int` `yywrap() ` `{ ` `    ``return` `1; ` `} `

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