Python program to validate email address

Email validation is a common requirement in web applications. We need to check whether an email address follows the correct format and character rules. Python's re module provides powerful regular expression capabilities for this task.

Email Validation Rules

A valid email must follow these conditions ?

• Format must be username@company.domain

• Username can contain letters, numbers, dashes, and underscores

• Company name can contain letters and numbers only

• Domain can contain lowercase letters only

• Domain extension length must be 1-3 characters

Using Regular Expression Pattern

We create a regex pattern that matches all validation rules ?

import re

def validate_email(email):
    # Pattern breakdown:
    # ^[a-zA-Z0-9-_]+ - Username: letters, numbers, dash, underscore
    # @ - Literal @ symbol
    # [a-zA-Z0-9]+ - Company: letters and numbers
    # \. - Literal dot (escaped)
    # [a-z]{1,3}$ - Domain: 1-3 lowercase letters at end
    pattern = "^[a-zA-Z0-9-_]+@[a-zA-Z0-9]+\.[a-z]{1,3}$"
    
    if re.match(pattern, email):
        return True
    return False

# Test with valid email
email1 = "popular_website15@company123.com"
print(f"'{email1}' is valid: {validate_email(email1)}")

# Test with invalid emails
email2 = "invalid@company.DOMAIN"  # Domain not lowercase
email3 = "user@.com"               # No company name
email4 = "user@company.toolong"    # Domain too long

print(f"'{email2}' is valid: {validate_email(email2)}")
print(f"'{email3}' is valid: {validate_email(email3)}")
print(f"'{email4}' is valid: {validate_email(email4)}")

'popular_website15@company123.com' is valid: True
'invalid@company.DOMAIN' is valid: False
'user@.com' is valid: False
'user@company.toolong' is valid: False

Alternative Approach with Detailed Validation

For more detailed error reporting, we can validate each part separately ?

import re

def detailed_email_validation(email):
    # Split email into parts
    if '@' not in email or email.count('@') != 1:
        return False, "Must contain exactly one @ symbol"
    
    username, domain_part = email.split('@')
    
    if '.' not in domain_part or domain_part.count('.') != 1:
        return False, "Domain must contain exactly one dot"
    
    company, domain = domain_part.split('.')
    
    # Validate username
    if not re.match("^[a-zA-Z0-9-_]+$", username):
        return False, "Username contains invalid characters"
    
    # Validate company
    if not re.match("^[a-zA-Z0-9]+$", company):
        return False, "Company name contains invalid characters"
    
    # Validate domain
    if not re.match("^[a-z]{1,3}$", domain):
        return False, "Domain must be 1-3 lowercase letters"
    
    return True, "Valid email"

# Test different emails
test_emails = [
    "user123@company.com",
    "user@company.COM",
    "user$@company.com",
    "user@company123.co"
]

for email in test_emails:
    is_valid, message = detailed_email_validation(email)
    print(f"'{email}': {message}")

'user123@company.com': Valid email
'user@company.COM': Domain must be 1-3 lowercase letters
'user$@company.com': Username contains invalid characters
'user@company123.co': Valid email

Pattern Explanation

Conclusion

Regular expressions provide an efficient way to validate email formats. Use re.match() with a proper pattern to check all validation rules in one step. For complex validation scenarios, consider breaking down the validation into separate checks for better error reporting.