// Code
%option noyywrap yylineno c++
%{
        #include <iostream>
%}
%%
"<"[A-Za-z]*">" { std::cout << YYText(); }
%%
int main () {
    yylex();
    return 0;
}

// Execution
$ flex test.l
$ gcc -o test lex.yy.cc 
test.l: In function 'int main()':
test.l:10: error: 'yylex' was not declared in this scope