input: Write a python function to validate and format a string input to match a specific pattern using regex.
  assert validate_input("hello") == "Hello"
  assert validate_input("WORLD") == "World"
  assert validate_input("Python123") == "Error: Invalid input"
  assert validate_input("testCase") == "Testcase"
output: import re
  
  def validate_input(s):
      if re.match("^[a-zA-Z]+$", s):
          return s.capitalize()
      else:
          return "Error: Invalid input"