added Lex file.

src/lex.l

+%{
+#include <stdio.h>
+#include "y.tab.h"
+%}
+
+%%
+
+[a-z0-9$._]+:       { return LABEL; }      /* Label */
+\$[a-z0-9._]        { return ARG; }        /* Argument of instruction */
+^[a-z.]+            { return INSTR; }      /* Instruction */
+\.[^\n]*            { return DIRECTIVE; }  /* Assembly */
+\,                  { return COMMA; }      /* Comma */
+#[^\n]*             { return COMMENT; }    /* Comment */
+\n                  { return NL; }         /* New line */
+	
+[\s\t]+    {;}