input: Write a function to extract all public email addresses from a given string using regular expression. The function should return a list of all found email addresses.
  
  import re
  
  def extract_emails(text):
      emails = re.findall(r"[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+", text)
      return emails
  
  assert extract_emails("My email is john@example.com and yours is john@gmail.com") == ['john@example.com', 'john@gmail.com']
  assert extract_emails("Please contact support@test.com for assistance.") == ['support@test.com']
  assert extract_emails("There are no valid emails here.") == []
  assert extract_emails("Email1: private-email@example.co and public-email@public.com") == ['private-email@example.co', 'public-email@public.com']
  assert extract_emails("") == []
output: def extract_emails(text):
      emails = re.findall(r"[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+", text)
      return emails