cd1.flex

%option noyywrap
%%
boolean|float|int|if|char printf("keywords");
[0-9][0-9]* printf("constants");
[a-z_A-Z][a-z_A-z0-9]* printf("identifiers");
\\(n|r|t) printf("escape sequence %s\n", yytext);
%%

int main()
{
    yylex();
}