# Validating Postal Codes with Regex: A HackerRank Solution

## Introduction

Validating postal codes is a crucial task for ensuring the accurate and efficient delivery of mail. In this article, we’ll focus on a HackerRank solution for validating postal codes with regex.

A valid postal code must meet two specific requirements: it must be a number within a given range, and it must not contain more than one alternating repetitive digit pair.

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We’ll explain the concept of alternating repetitive digits and guide you through the process of implementing the validation in Python using regular expressions (regex).

## Understanding Alternating Repetitive Digits

Before we dive into the regex implementation, let’s clarify what alternating repetitive digits mean. An alternating repetitive digit pair consists of two equal digits separated by exactly one other digit.

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For example, in the postal code “121426,” the digit “2” repeats immediately after the digit “1,” forming an alternating repetitive digit pair.

## The Postal Code Range

The postal code must be a number within a specific range, inclusive of the lower and upper bounds. While the actual range values may vary depending on the country or region, for the purpose of this example, let’s consider the range from 100000 to 999999 inclusive.

## What is Regex?

Regular expressions (regex) are powerful tools for pattern matching and text manipulation. They provide a concise and flexible way to search, extract, and validate strings based on specific patterns.

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In our case, regex will help us check for alternating repetitive digit pairs and validate the postal code range.

## The HackerRank Solution

Now, let’s explore the step-by-step guide to implementing regex for postal code validation in Python using the HackerRank solution.

## Step-by-Step Guide to Implementing Regex for Postal Code Validation in Python

### 1. Import the Required Libraries

To work with regex in Python, we need to import the `re` module.

``import re``

Explanation: In this line of code, we import the `re` module, which is a standard Python library that provides support for working with regular expressions.

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### 2. Define the Regex Pattern

To check for alternating repetitive digit pairs, we can use the following regex pattern:

``regex_pattern = r'^(\d)(?=(\d)\1)(\d)\2(?!\2)\d{3}\$'``

Explanation: In this line of code, we define the regex pattern that will be used to validate the postal code. Let’s break down the components of the pattern:

• `^` asserts the start of the string.
• `(\d)` captures the first digit of the postal code.
• `(?=(\d)\1)` is a positive lookahead that matches the second digit if it is the same as the first captured digit.
• `(\d)\2` captures the third digit, which must be the same as the second digit.
• `(?!\2)` is a negative lookahead that ensures the fourth digit is not the same as the third captured digit.
• `\d{3}` matches the remaining three digits in the postal code.
• `\$` asserts the end of the string.

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### 3. Validate the Postal Code

Let’s define a function that takes a postal code as input and returns whether it is valid or not.

``````def is_valid_postal_code(postal_code):
return bool(re.match(regex_pattern, postal_code))``````

Explanation: In this code block, we define a function called `is_valid_postal_code`. The function takes a postal code as input and uses the `re.match()` method to check if the postal code matches the regex pattern defined earlier.

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If the postal code matches the pattern, the function returns `True`, indicating that the postal code is valid. Otherwise, it returns `False`, indicating that the postal code is invalid.

### 4. User Input and Output

Now, let’s ask the user to input a postal code, validate it using the defined function, and display the result.

``````def main():
postal_code = input("Enter a postal code: ")
if is_valid_postal_code(postal_code):
print("The postal code is valid.")
else:
print("The postal code is invalid.")

if __name__ == "__main__":
main()``````

Explanation: In this code block, we define a `main()` function that handles user input and output. The user is prompted to enter a postal code using the `input()` function.

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The entered postal code is then passed to the `is_valid_postal_code()` function for validation. Depending on the validation result, the program prints whether the postal code is valid or invalid.

## FAQs

1. What if the postal code is outside the specified range?

If the postal code is outside the range from 100000 to 999999, it will be considered invalid.

2. Can regex handle postal codes from different countries?

Yes, the regex pattern provided in the article can handle postal codes with alternating repetitive digit pairs from various countries.

3. Are all postal codes with alternating repetitive digit pairs valid?

No, postal codes with more than one alternating repetitive digit pair are considered invalid.

4. Can I use the provided Python code for other regex validations?

Absolutely! The regex pattern can be modified to validate other strings based on specific patterns.

5. What if my country uses a different postal code format?

You can adapt the regex pattern to match the postal code format of your country by adjusting the capturing groups and quantifiers accordingly.

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## Conclusion

Validating postal codes with regex is an essential task in ensuring accurate mail delivery. In this article, we’ve explored the concept of alternating repetitive digit pairs and the requirements for a valid postal code.

We’ve also provided a step-by-step guide to implementing postal code validation in Python using regex with a HackerRank solution.

By understanding each line of code and using regular expressions, you can efficiently validate postal codes and ensure the integrity of your mail data.

Remember to test your implementation thoroughly and consider adjusting the regex pattern as per specific postal code requirements in different regions.

Now it’s your turn to try the validation yourself! Enter a postal code and see if it’s valid or not.