input: Write a function to develop a regular expression in Python that matches all numeric strings. The function should return True if the input string is a valid numeric string and False otherwise. If the input string is empty, the function should return False.
  assert is_numeric_string('123456789')==True
  assert is_numeric_string('abc123')==False
  assert is_numeric_string('')==False
  assert is_numeric_string('123.45')==False
  assert is_numeric_string('123e45')==False
output: def is_numeric_string(s):
      import re
      pattern = re.compile(r'^\d+$')
      return bool(pattern.match(s))