Getting: warning, rule cannot be matched

Question

I am working on building a lexical and syntax analyzer. I am getting the following warning when I try to use flex with my .l file. ``` littleDuck.l:26: warning, rule cannot be matched ``` Rule 26 is the one that starts with {cteI}, my rules section is the following: ``` [ ] ; {RW} {return RESERVED;} {id} {return ID;} {ops} {return OPERATOR;} {seps} {return SEPARATOR;} {cteI} {yylval.ival = atoi(yytext); return INT;} {cteF} {yylval.fval = atof(yytext); return FLOAT;} {ctestring} {yylval.sval = strdup(yytext); return STRING;} . ; ``` Also, my definitions section is this: ``` RW program|var|int|float|print|else|if id ([a-z]|[A-Z)([a-z]|[A-Z]|[0-9])* ops "="|"<"|">"|"<>"|"+"|"-"|"/"|"*" seps ":"|","|";"|"{"|"}"|"("|")" cteI [0-9]+ cteF {cteI}(\.{cteI}((e|E)("+"|"-")?{cteI})?)? ctestring (".*") ``` Why is this warning appearing, and how can I modify my file so it will not appear?

Accepted Answer

The warning tells you that anything that might be matched by `{cteI}` will always be matched by some earlier rule. In your case, it indicates that a rule doesn't match what you expect it does, probably due to a typo. In your case, its the `{id}` rule, which you define as:

```
([a-z]|[A-Z)([a-z]|[A-Z]|[0-9])*
```

Notice the nesting of the parentheses and square brackets. Adding spaces for clarity, this is

```
( [a-z] | [A-Z)([a-z] | [A-Z] | [0-9] )*
```

This will match any sequence of letters, digits, or the characters `(` `)` or `[`. You probably meant:

```
([a-z]|[A-Z])([a-z]|[A-Z]|[0-9])*
```

which can be more clearly written as

```
[a-zA-Z][a-zA-Z0-9]*
```

Getting: warning, rule cannot be matched

Solution

Problem