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Working with this example string "E-112233-555-11 | Bläh - Bläh" with the following code leeds under python 2.7.10 (OSX) to an exception whereas the same code works under python 3.5.1 (OSX).
s = "E-112233-555-11 | Bläh - Bläh"
expr = re.compile(r"(?P<p>[A-Z]{1}-[0-9]{0,}(-[0-9]{0,}(-[0-9]{0,})?)?)?(( [|] )?(?P<a>[\s\w]*)?)? - (?P<j>[\s\w]*)?",re.UNICODE)
res = re.match(expr,s)
a = (res.group('p'), res.group('a'), res.group('j'))
print(a)
When I change the first umlaut in "Bläh" from ä to ü it works as expected on python 2 and 3. A change from ä to ö however leeds to a crash again.
Ideas?

First, in the context of Python a crash means a core dump or an analogue on Windows. In this case the code just works not as you expected.
The short answer: s should be a unicode.
In your code "ä" is encoded as 8-bit string '\xc3\xa4'. When matched, every bytes is independently expanded to Unicode range. The first byte becomes u'\xc3' = u'Ã', the second byte becomes u'¤', non-alphanumeric. '[\s\w]*' doesn't match u'ä'.
"ü" is encoded as 8-bit string '\xc3\xbc'. The second byte becomes u'1⁄4', numeric. '[\s\w]*' matches u'Ã1⁄4'.

Thx for your explanation. You explained why [\s\w] didn't match for "ä". In my situation it didn't matches for the first "ä" but the second time I used [\s\w] in the same regex it matched at the second "ä". What's the explanation for this?

Sorry, I don't understand you. If the regex failed to match the first "ä", it can't match the second "ä". Do you have an example?

When I replace the first "ä" with a random letter the untouched expression has not problems to match the second word which contains also an "ä"
s = "E-112233-555-11 | Bläh - Bläh" #untuched string
s = "E-112233-555-11 | Bloh - Bläh" #string where the first ä is replaced by an "o"

Because "[\s\w]*" matches only a part of "Bläh": "Bl\xc3".