input: Write a python function to validate email addresses using regular expressions.
  assert validate_email("test@example.com")==True
  assert validate_email("test.email@example.com")==True
  assert validate_email("test.email+alex@leetcode.com")==True
  assert validate_email("plainaddress")==False
  assert validate_email("@missingusername.com")==False
output: def validate_email(email):
      import re
      pattern = r'^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$'
      return re.match(pattern, email) is not None