Lab 8: Regular Expressions in C++ and Linux

Instructions

First read this page then start working on lab with the GitHub classroom link below.
Use the code in the GitHub repository for this lab.
Github Classroom Link: https://classroom.github.com/a/HiREPVtw

Objective

Understand the basics of regular expressions and how to use them. Through practical examples, learn how regex can be helpful for data related tasks like validation and searching.

Preliminaries

There are three functions that are a part of the <regex> library. https://en.cppreference.com/w/cpp/regex

regex_match
- Attempts to match a regular expression to an entire character sequence

    string email = "dbalash@richmond.edu";

    // Regular expression pattern for email validation
    regex pattern{R"([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})"};

    if (regex_match(email, pattern)) 
    {
        cout << "Valid email address!" << endl;
    } 

regex_search
- Attempts to match a regular expression to any part of a character sequence

string stringToSearch = "Hello, World!";
smatch stringMatches;
regex regexPattern(R"((\w+))");  // Match a word

if (regex_search(stringToSearch, stringMatches, regexPattern)) 
{
    cout << "First word: " << stringMatches[0] << endl;
}

regex_replace
- Replaces occurrences of a regular expression with formatted replacement text

string stringToModify = "cats are great, I love cats!";
regex regexPattern(R"(cats)");
string replacement = "dogs";
string result = regex_replace(stringToModify, regexPattern, replacement);
cout << result;  // "dogs are great, I love dogs!"

Part 1: Password Strength Checker

Write a program that checks the strength of a user’s password.
The password is considered strong if it:
- Contains at least 6 characters
- Contains at most 20 characters.
- Contains at least one lowercase letter
- Contains at least one uppercase letter
- Contains at least one digit
- Does not contain three repeating characters in a row, like “aaa”
Hint: You might need multiple regex patterns to verify these conditions.
Hint: For the three repeating characters in a row.
- See Lesson 15 on back referencing
- https://regexone.com/lesson/misc_meta_characters
Start with the code in the file password.cpp
Implement the isStrongPassword function.

Part 2: Analyzing a web server log file

Imagine you’re a system administrator for a web server. You need to analyze the server’s access logs to monitor and respond to specific types of requests. Let’s consider a simplified version of the Common Log Format, which looks like this:

Excerpt from the file webserver.log available in repository.

0.0.1 - a3b2c1d [10/Oct/2023:13:55:36 -0700] "GET /index.html HTTP/1.0" 200 2326
0.0.5 - f4g5h6i7 [10/Oct/2023:13:56:40 -0700] "POST /submit-data HTTP/1.0" 503 1226
168.1.102 - j8k9l0m [10/Oct/2023:14:02:20 -0700] "GET /images/photo.jpg HTTP/1.1" 404 762

Each log entry includes:

Client IP address
User ID (if HTTP authentication was performed)
Date, time, and timezone
Request line from the client in double quotes
Status code returned to the client
Size of the object returned to the client

For each of the following, put the Linux bash shell command that you used along with the results in the README.md file for this lab.

Do the following:

Extract all 404 errors
- Find all requests that resulted in a 404 status code (File Not Found)
Find all image requests
- Find all requests made to image files (.jpg, .png, and .gif)
Identify all POST requests
- Find all POST requests made to the server
List all unique IPs
- List all unique IP addresses that have accessed the server

Hint: Use the grep -E command to search the log.

grep -E 'YOUR REGEX HERE' webserver.log

Hint: Use the grep -E -o command to search the log and only return the matched string.

grep -E -o 'YOUR REGEX HERE' webserver.log

Hint: You can pipe the output of your grep command to other unix commands such as sort or uniq.

grep -E -o 'YOUR REGEX HERE' webserver.log | sort | uniq

NOTE: the \d does not work with grep, use [0-9] instead.

Part 3: Searching for Failed Authentication Attempts in an OpenSSH Log

Imagine you’re managing a server and you’re monitoring the SSH log for security. You have a log file named auth.log with entries like:

Excerpt from the file auth.log available in repository.

Dec 10 07:28:08 LabSZ sshd[24247]: Failed password for root from 112.95.230.3 port 55618 ssh2
Dec 10 07:28:08 LabSZ sshd[24247]: Received disconnect from 112.95.230.3: 11: Bye Bye [preauth]
Dec 10 07:28:08 LabSZ sshd[24249]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=112.95.230.3  user=root
Dec 10 07:28:10 LabSZ sshd[24249]: Failed password for root from 112.95.230.3 port 57138 ssh2
Dec 10 07:28:10 LabSZ sshd[24249]: Received disconnect from 112.95.230.3: 11: Bye Bye [preauth]

For each of the following, put the Linux bash shell command that you used along with the results in the README.md file for this lab.

You want to identify all failed login attempts, and for this purpose, you specifically want to extract:
- The timestamp.
- The user for which the login failed.
- The IP address from which the login attempt was made.

Using grep, extract pertinent information about failed SSH login attempts from the auth.log, helping in identifying potential unauthorized access attempts.

Example output:

Dec 10 09:11:26 LabSZ sshd[24437]: Failed password for invalid user admin from 185.190.58.151 port 44155 ssh2
Dec 10 09:11:28 LabSZ sshd[24443]: Failed password for invalid user user from 103.99.0.122 port 62581 ssh2
Dec 10 09:11:31 LabSZ sshd[24445]: Failed password for root from 103.99.0.122 port 49486 ssh2

Part 4: Search and Replace

Use C++ and the regex library to search and replace from an HTML string.

Tasks:

Edit the replace.cpp file in the Part4 directory in the repository
Modify the provided C++ code to replace all links starting with /local/ with links starting with https://newsite.com/

Hint: Use a regex pattern and the regex_replace function.

Expected Output: When the modified code runs, it should print the HTML with the updated links.

<!DOCTYPE html>
<html>
<head>
    <title>Sample Page</title>
</head>
<body>
    <h1>Welcome to the Sample Page</h1>
    <p>Here are some links:</p>
    <ul>
        <li><a href="https://www.example.com">Example</a></li>
        <li><a href="http://www.testsite.com/page1">Test Site</a></li>
        <li><a href="https://newsite.com/link1">Local Link 1</a></li>
        <li><a href="https://newsite.com/link2">Local Link 2</a></li>
    </ul>
    <p>Thank you for visiting!</p>
</body>
</html>