Python substitute a word for a word and the next concatenated
<p>I want to be able to take in a string and if r'\snot\s' is located, essentially concatenate 'not' and the next word (replacing the white space in between with an underscore).</p>

<p>So if the string is </p>

<p>string="not that my name is Brian and I am not happy about nothing"</p>

<p>The result after a regular expression would be:</p>

<p>'not_that my name is Brian and I am not_happy about nothing'</p>

<p>(not in nothing is not touched).</p>

<p>I need to locate 'not' that is either seperated by white space or at the start of a sentence and then join it to '_' and the next word.</p>