Simple scanner

scanner.l

%{
#include <stdio.h>
#include <stdlib.h>

enum {
    End,
    Comma,
    Equals,
    Semicolon,
    Number,
    Identifier,
};
%}

digit   [0-9]
idstart [a-zA-Z_]
idcont  [a-zA-Z_0-9]

%option noyywrap
%option yylineno
%%

[ \t\n\r]+          { }
"//".*              { }

","                 { return Comma; }
"="                 { return Equals; }
";"                 { return Semicolon; }

[-+]?{digit}+       { return Number; }
{idstart}{idcont}*  { return Identifier; }

.                   { printf("line %d: unexpected character: '%s'\n", yylineno, yytext); exit(1); }

%%

const char *const TokNames[] = {
  [Comma] = "Comma",
  [Equals] = "Equals",
  [Semicolon] = "Semicolon",
  [Number] = "Number",
  [Identifier] = "Identifier",
};

int main(int argc, char **argv) {
  while (1) {
    int tok = yylex();
    if (tok == 0) {
      break;
    }
    printf("%s '%s'\n", TokNames[tok], yytext);
  }
}