Fixed OFFSET syntax error.

src/lex.l

@@ -13,7 +13,7 @@ int     [1-9][0-9]*
 {reg}               { yylval.sval = yytext; return REG; }       /* Registry address */
 {word}:             { yylval.sval = yytext; return LABEL; }     /* Label */
 {int}               { yylval.ival = atoi(yytext); return INT; } /* Integer */
-[0-9]+(\({reg}\))?  { yylval.sval = yytext; return OFFSET; }    /* Registry offset */
+{int}(\({reg}\))?   { yylval.sval = yytext; return OFFSET; }    /* Registry offset */
 [a-z\.]+            { yylval.sval = yytext; return INSTR; }     /* Instruction */
 {word}              { yylval.sval = yytext; return REF; }       /* Label reference */
 [,]                 { return COMMA; }                           /* Comma */

src/lex_out.y

@@ -23,6 +23,7 @@ symb:
     | LABEL {printf("Found a label: %s\n", $1);}
     | REF {printf("Found a label reference: %s\n", $1);}
     | INT {printf("Found an integer: %d\n", $1);}
+    | OFFSET {printf("Found an offset registry address: %s\n", $1);}
     | INSTR {printf("Found an instruction: %s\n", $1);}
     | COMMA {printf("Found a comma\n");}
     | NL {printf("Found a newline\n");}