input: Write a function to create a regular expression pattern that matches all numeric strings. The function should use the Python `re` module and return the pattern. Additionally, it should include an assert statement to test the regex pattern on a given input string.
  
  
  def create_numeric_regex():
      import re
      pattern = '^[0-9]+$'
      assert re.match(pattern, '123456789')
      return pattern
  
  assert create_numeric_regex().pattern == '^[0-9]+$'  # Output: '^[0-9]+$'
  
output: def create_numeric_regex():
      import re
      pattern = '^[0-9]+$'
      return pattern