title: Python regexp obtain not matched content
body: <p>I need to parse a line of text and separate in parts and add it to a list, thing that i was able to do with the help of re.parse('regexp'). The thing is that i get some text that i dont want that match on this, but i need to know where is it, and how to detect it and of course what is it, to show an error.</p>
  
  <p>the code matches  and filters out all perfectly, the thing is i need to filter out the 12 and the 32 that are not matching the regexp</p>
  
  <pre>
  import re 
  
  str = '12 32 455c 2v 12tv v 0.5b -3b -b+b-3li b-0.5b 3 c -3 ltr'
  a=re.compile(r'[+-]?[0-9]*\.[0-9]+\s*[a-z]+|[+-]?[0-9]*\s*[a-z]+')
  
  r=a.findall(str)
  print (r)
  </pre>
  
  <p>Initial String:</p>
  
  <pre>
  str= '12 32 455c 2v 12tv v 0.5b -3b -b+b-3li b-0.5b 1 3 c -3 ltr'
  list parsed, correctly
  ['455c', '2v', '12tv', ' v', '0.5b', '-3b', '-b', '+b', '-3li', ' b', '-0.5b', '3 c', '-3 ltr']
  list that i need as well and any other string not matched ie: (/%&$%)
  [12, 32, 1]
  </pre>