import re
s = 'Total revenue for 201603 is 3000 €'
rx = re.compile(r'^(?=\d+(?:[_-]\d+)?$)[\d_-]{6,7}$')
res = [x for x in s.split() if rx.search(x)]
if len(res):
	print(res[0])

# Pure regex approach:
rx = re.compile(r'(?<!\S)(?=\d+(?:[_-]\d+)?(?!\S))[\d_-]{6,7}(?!\S)')
res = rx.search(s)
if res:
	print(res.group())