2. Implement a Lexical Analyzer for a given program using Lex Tool.

 Let's assume we have a simple programming language with keywords `if`, `else`, `while`, and identifiers (variable names) consisting of letters and digits. We want to tokenize a given program written in this language.

Here's the Lex specification file (`lexer.l`) along with explanations:

```lex

%{

#include <stdio.h>

%}

%option noyywrap

%%

if      { printf("IF\n"); }

else    { printf("ELSE\n"); }

while   { printf("WHILE\n"); }

[a-zA-Z][a-zA-Z0-9]* { printf("IDENTIFIER: %s\n", yytext); }

[ \t\n]  ; // Skip whitespace

.        { printf("UNKNOWN CHARACTER: %s\n", yytext); }

%%

int main() {

    yylex();

    return 0;

}

```

Explanation of each section:

- `%{ ... %}`: This section is used for including any necessary header files and declaring global variables or definitions. In this case, we include `stdio.h` for printing messages.

- `%option noyywrap`: This option indicates that the `yywrap` function won't be used. It's commonly used to signal the end of input in Lex programs.

- `%%`: This delimiter separates the Lex rules from the user code.

- `if`, `else`, `while`: These are keywords in our language. For each keyword, we specify a regular expression followed by an action to be taken when a match is found. In this case, we print the corresponding token name.

- `[a-zA-Z][a-zA-Z0-9]*`: This regular expression matches identifiers. It starts with a letter (uppercase or lowercase) and can be followed by letters or digits. When an identifier is matched, we print its value using `yytext`.

- `[ \t\n]`: This regular expression matches whitespace characters (spaces, tabs, newlines). We skip these characters.

- `.`: This regular expression matches any character that didn't match any of the previous patterns. When an unknown character is encountered, we print its value using `yytext`.

- The final section (`main()`) initializes the Lexical Analyzer using `yylex()`.

To compile and run the program:

1. Save the Lex specification in a file named `lexer.l`.

2. Open a terminal and navigate to the directory containing `lexer.l`.

3. Run the following commands:

   - `lex lexer.l` (compiles the Lex specification)

   - `gcc lex.yy.c -o lexer -ll` (compiles the Lex-generated code)

   - `./lexer` (runs the program)

Now, you can provide input text (your program) to the compiled lexer, and it will tokenize the input and display the corresponding tokens.