input: Write a function to develop a regular expression in Python to match all numeric strings. The function should take a string as input and return True if the string is a valid numeric string, otherwise return False.
  assert match_numeric_string('123456789')==True
  assert match_numeric_string('123a456789')==False
  assert match_numeric_string('0')==True
  assert match_numeric_string('')==False
  assert match_numeric_string('123.45')==False
output: def match_numeric_string(s):
      import re
      return bool(re.fullmatch(r'\d+', s))