import re
txt = 'Την 02/12/2013 καταχωρήθηκε στο Γενικό Εμπορικό Μητρώο της Υπηρεσίας Γ.Ε.ΜΗ. του Επιμελητηρίου Βοιωτίας, με κωδικόαριθμό καταχώρισης Κ.Α.Κ.: 110035'
p = re.compile(r'''Την\s? # matches Την with a possible space afterwards
(?P<KEK_date>\d{2}/\d{2}/\d{4}) #matches a date of the given format and captures it with a named group
.+ # Allow for an arbitrary sequence of characters 
(?:κωδικ.\s?αριθμ.\s?καταχ.ριση.|κ\.?α\.κ\.:?)\s+ # defines two lookaheads, either of which suffices
(?P<KEK_number>\d+) # captures a sequence of numbers''', re.I | re.X)
print(p.findall(txt))

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