text = 'yeah, but no, but yeah, but no, but yeah'
 text.replace('yeah', 'yep')
 'yep, but no, but yep, but no, but yep'

 text = 'Today is 11/27/2012. PyCon starts 3/13/2013.'
 import re
 re.sub(r'(\d+)/(\d+)/(\d+)', r'\3-\1-\2', text)
 'Today is 2012-11-27. PyCon starts 2013-3-13.'

 rom calendar import month_abbr
 def change_date(m):
 ... mon_name = month_abbr[int(m.group(1))]
 ... return '{} {} {}'.format(m.group(2), mon_name, m.group(3))
 ...
 datepat.sub(change_date, text)
 'Today is 27 Nov 2012. PyCon starts 13 Mar 2013.'f

 newtext, n = datepat.subn(r'\3-\1-\2', text)
 newtext
 'Today is 2012-11-27. PyCon starts 2013-3-13.'
 n
 2