code: def _replace(variables, match):
      
      expression = match.group(1)
  
      # Look-up chars and functions for the specified operator
      (prefix_char, separator_char, split_fn, escape_fn,
       format_fn) = operator_map.get(expression[0], defaults)
  
      replacements = []
      for key, modify_fn, explode in split_fn(expression):
          if key in variables:
              variable = modify_fn(variables[key])
              replacement = format_fn(
                  explode, separator_char, escape_fn, key, variable)
              replacements.append(replacement)
      if not replacements:
          return ''
      return prefix_char + separator_char.join(replacements)
docs: Return the appropriate replacement for `match` using the passed variables