input: Write a function to create a regular expression that matches all numeric strings. The function should return the regular expression pattern as a string. If no input string is provided, the function should return the pattern for numeric strings.
  assert create_numeric_regex()=='[0-9]+'
  assert create_numeric_regex().match('123456789')!=None
  assert create_numeric_regex().match('abc123')==None
  assert create_numeric_regex().match('')==None
  assert create_numeric_regex().match('123abc')==None
output: def create_numeric_regex():
      return '[0-9]+'