input: Write a python function to filter valid names using regular expressions.
  assert filter_names(['Alex', 'Mark', 'John21', '12345']) == ['Alex', 'Mark']
  assert filter_names(['Anna', 'Bob', 'Cathy123', 'D4n']) == ['Anna', 'Bob']
  assert filter_names(['Eve', 'Frank', 'George', '123']) == ['Eve', 'Frank', 'George']
output: import re
  
  def filter_names(names):
      pattern = re.compile(r'^[A-Za-z]+$')
      return [name for name in names if pattern.match(name)]