input: Develop a function to eliminate all whitespace characters from an input string using regular expressions.
  assert remove_whitespace_chars(' Google    Flutter ') == 'GoogleFlutter'
  assert remove_whitespace_chars(' Google    Dart ') == 'GoogleDart'
  assert remove_whitespace_chars(' iOS    Swift ') == 'iOSSwift'
output:     import re
      return re.sub(r'\s+', '', input_string)