import re
line ="[u'INVOICE# SMR/0038 f\"', u'', u'', u'']"
pattern = re.compile('invoice#\s+([A-Za-z]{3}/\d{1,5})', re.I)
print( re.findall(pattern, line) )
# get the first match
m = pattern.search(line)
if m:
print(m.group(1))
aW1wb3J0IHJlCmxpbmUgPSJbdSdJTlZPSUNFIyBTTVIvMDAzOCBmXCInLCB1JycsIHUnJywgdScnXSIKcGF0dGVybiA9IHJlLmNvbXBpbGUoJ2ludm9pY2UjXHMrKFtBLVphLXpdezN9L1xkezEsNX0pJywgcmUuSSkKcHJpbnQoIHJlLmZpbmRhbGwocGF0dGVybiwgbGluZSkgKQojIGdldCB0aGUgZmlyc3QgbWF0Y2gKbSA9IHBhdHRlcm4uc2VhcmNoKGxpbmUpCmlmIG06CglwcmludChtLmdyb3VwKDEpKQ==