Looks for any of the following pull request formats in the description field:

            pr12345, pr 2345, PR2345, PR 2345

def pull_requests(self):
        '''
        Looks for any of the following pull request formats in the description field:

            pr12345, pr 2345, PR2345, PR 2345
        '''
        pr_numbers = re.findall(r"[pP][rR]\s?[0-9]+", self.description)
        pr_numbers += re.findall(re.compile("pull\s?request\s?[0-9]+", re.IGNORECASE), self.description)

        # Remove Duplicates
        pr_numbers = [re.sub('[^0-9]','', p) for p in pr_numbers]
        return pr_numbers