input: Write a function to determine if a given string consists only of numeric characters using a regular expression. The function should return True if the string is numeric, and False otherwise. If the given string is empty, it should return False.
  
  import re
  
  def is_numeric_string(s):
      pattern = '^[0-9]+$'
      if not s:
          return False
      return re.match(pattern, s) is not None
  
  assert is_numeric_string("123456789") == True
  assert is_numeric_string("1234abcd") == False
  assert is_numeric_string("") == False
  assert is_numeric_string("0") == True
  assert is_numeric_string("1234567890") == True
output: import re
  
  def is_numeric_string(s):
      pattern = '^[0-9]+$'
      if not s:
          return False
      return re.match(pattern, s) is not None