input: Write a function to check if a given string contains only numeric characters using regular expressions. If the string is numeric return True, otherwise return False.
  import re
  
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)
  
  assert is_numeric_string('123456789')==True
  assert is_numeric_string('123abc')==False
  assert is_numeric_string('abc123')==False
  assert is_numeric_string('')==False
  assert is_numeric_string('0')==True
output: import re
  
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)