I got some text extracted and wish to clean it up by RegEx.

I have learned basic RegEx but not sure how to build this one:

str = '''
this is 
a line that has been cut.
This is a line that should start on a new line
'''

should be converted to this:

str = '''
this is a line that has been cut.
This is a line that should start on a new line
'''

This r'\w\n\w' seems to catch it, but not sure how to replace the new line with space and not touch the end and beginning of words

You can use this lookbehind regex for re.sub:

>>> str = '''
... this is
... a line that has been cut.
... This is a line that should start on a new line
... '''
>>> print re.sub(r'(?<!\.)\n', '', str)
this is a line that has been cut.
This is a line that should start on a new line
>>>

RegEx Demo

(?<!\.)\n matches all line breaks that are not preceded by a dot.

If you don't want a match based on presence of dot then use:

re.sub(r'(?<=\w\s)\n', '', str)

RegEx Demo 2