LETTER [a-zA-Z]
DIGIT [0-9]
%%
begin {printf("Recognized KeyWORD:%s\n",yytext);}
if    {printf("Recognized KeyWORD:%s\n",yytext);}
{LETTER}({LETTER}|{DIGIT})*{
           printf("Recognized ID:%s\n",yytext);}
%%
main()
{
yylex();
}

ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: toplevel: procedure `program' does not exist (DWIM could not correct goal)
   Exception: (3) program ?    Exception: (3)  ? ERROR: program/0: stream `debugger_write_options' does not exist
   Exception: (3)  ? % Break level 1

ERROR: Stream user_input:15:0 Syntax error: Unexpected end of file
% Exit break level 1
   Exception: (3)  ? EOF: exit