input: Write a function to check if a given string is a valid numeric string using a regular expression. If the string matches the pattern of a numeric string, return True, otherwise return False.
  
  assert is_numeric_string('123456789') == True
  assert is_numeric_string('abc123') == False
  assert is_numeric_string('123.456') == False
  assert is_numeric_string('') == False
  assert is_numeric_string('0') == True
output: import re
  
  def is_numeric_string(s):
      return bool(re.fullmatch(r'\d+', s))