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Lex program to identify the identifier

  • Last Updated : 30 Apr, 2019

Lex is a computer program that generates lexical analyzers and was written by Mike Lesk and Eric Schmidt. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lex in the C programming language.

In C, an identifier must begin with either an alphabet or underscore, it can not begin with a digit or any other special character, moreover digits can come after.


gfg : valid identifier
123 : invalid identifier
_abc12 : valid identifier
#abc : invalid identifier

Let’s see the lex program to determine whether input is an identifier or not.

/*lex code to determine whether input is an identifier or not*/
% {
#include <stdio.h>
    / rule section % %
    // regex for valid identifiers
    ^[a - z A - Z _][a - z A - Z 0 - 9 _] * printf("Valid Identifier");
// regex for invalid identifiers
^[^a - z A - Z _] printf("Invalid Identifier");
% %


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