7. Generate a YACC specification to recognize a valid identifier which starts with a letter followed by any number of letters or digits.

 here's a YACC specification that recognizes a valid identifier according to the given criteria:

```yacc

%{

#include <stdio.h>

%}

%token IDENTIFIER

%%

start: IDENTIFIER   { printf("Valid identifier: %s\n", $1); }

     ;

IDENTIFIER: [a-zA-Z][a-zA-Z0-9]*

     ;

%%

int main() {

    yyparse();

    return 0;

}

```

Explanation of each line of code:

1. `%{` and `%}`: These delimiters are used to include C code that will be inserted into the generated parser code. In this case, it's used to include the `<stdio.h>` header for printing.

2. `%token IDENTIFIER`: Declares the token type `IDENTIFIER`. This informs YACC that the parser will recognize and generate tokens of this type.

3. `%%`: Separates the YACC grammar rules from the C code.

4. `start: IDENTIFIER`: Defines the `start` rule. This rule states that the input should consist of a valid identifier.

5. `{ printf("Valid identifier: %s\n", $1); }`: This action is associated with the `start` rule. It's executed when the rule is recognized. `$1` refers to the first value on the right-hand side of the rule, which is the recognized identifier. This action prints the valid identifier.

6. `;`: Ends the `start` rule.

7. `IDENTIFIER: [a-zA-Z][a-zA-Z0-9]*`: This is the definition of the `IDENTIFIER` token. It follows the given criteria that an identifier starts with a letter and is followed by any number of letters or digits.

    - `[a-zA-Z]`: Represents a single letter (uppercase or lowercase).

    - `[a-zA-Z0-9]*`: Represents zero or more letters or digits after the initial letter.

8. `;`: Ends the `IDENTIFIER` token definition.

9. `%%`: Separates the grammar rules from the C code.

10. `int main() { yyparse(); return 0; }`: This is the main function. It calls the YACC-generated parser (`yyparse()`) to start parsing the input. After parsing, the program returns 0.

This YACC specification defines the grammar rules for recognizing valid identifiers and includes C code to print the recognized identifiers. Remember that YACC generates C code for the parser based on these specifications.