Lex: Avoid a quoted expression - regex

Question

I'm trying to build a lex program which should avoid a quoted sequence of characters. Something like not(":="). I've written this so far but can't seem to be able to get the desired output:
/* Definitions */
assgn ":="
symbol [^{assgn}]
%%
.
":=" {printf("Found - %s\n",yytext);}
{symbol} {printf("Error: Unmatched symbol \'%s\'\n", yytext);}
%
Input:
:= & | #
Output:
Found - :=
Error: Unmatched symbol ':'
Error: Unmatched symbol '='
Error: Unmatched symbol '&'
Error: Unmatched symbol '|'
Error: Unmatched symbol '#'
Desired output:
Found - :=
Error: Unmatched symbol '&'
Error: Unmatched symbol '|'
Error: Unmatched symbol '#'
I want that the program avoids the ":=" as a whole but it still reads the single character ":". How do I correct this?

Answer 1

The following little complete (f)lex program does precisely what is desired. flex will not rescan a matched token unless you ask it to (or, of course, there is some undefined behaviour in your code which corrupts flex's internal data structures.)
%{
#include <stdio.h>
%}
%option nodefault noyywrap noinput nounput
%%
[[:space:]]+ /* Ignore spaces */
":=" {printf("Found - %s\n",yytext);}
. {printf("Error: Unmatched symbol \'%s\'\n", yytext);}
With the above file stored in only:=.l, I do the following:
$ lex -o only:=.c only:=.l
$ gcc -o only:= -Wall only:=.c -lfl
$ ./only:=
:= & | #
Found - :=
Error: Unmatched symbol '&'
Error: Unmatched symbol '|'
Error: Unmatched symbol '#'
(-lfl adds a minimal definition of main())