import re
print re.search(r'd{4}-d{2}-d{2}', 'xxxx1990-12-20xxxx').group(0)
print re.search(r'd{4}-d{2}-d{2}Td{2}:d{2}:d{2}', 'xxxx2005-06-04T18:37:11xxxx').group(0)
print re.search(r'd{4}-d{2}-d{2}Td{2}:d{2}:d{2}.d{3}', 'xxxx2005-06-04T18:37:11.111xxxx').group(0)
pattern=re.compile(r'(d{4}-d{2}-d{2})((Td{2}:d{2}:d{2}|))((.d{3})|)')
print pattern.search('xxxx2005-06-04T18:37:11.111xxxx').group(0)