input: Write a function to extract all public email addresses from a given string. The function should return a list of email addresses found in the input string. If no email is found, it should return an empty list.
  assert extract_emails("My email is john@example.com and yours is john@gmail.com.") == ['john@example.com', 'john@gmail.com']
  assert extract_emails("No emails here.") == []
  assert extract_emails("") == []
output: import re
  
  def extract_emails(text):
      pattern = r'[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}'
      return re.findall(pattern, text)