%option noyywrap yylineno c++ debug
%x INSIDE
%{
        #include <iostream>
%}

LETTER [A-Za-z]

%%
<INITIAL>       "<"             BEGIN INSIDE;
<INITIAL>       .|\n            ;
<INSIDE>        ">"             BEGIN INITIAL;
<INSIDE>        {LETTER}*       { std::cout << YYText(); }
%%

int main() {
        FlexLexer* lexer = new yyFlexLexer();
        while (lexer->yylex() != 0)
                ;
        return 0;
}

ist156897@compiladores ~ $ flex test.l; g++ -o test lex.yy.cc; ./test < text.txt
test.l:11: unrecognized rule
test.l:11: unrecognized rule
test.l:11: unrecognized rule
test.l:11: unrecognized rule
test.l:13: unrecognized rule
test.l:13: unrecognized rule
test.l:13: unrecognized rule
test.l:13: unrecognized rule