I just went through some calculations for a (government) site that has the following rules:
[...]
Under the plausible assumption that very many people will start with a string of digits, continue with a string of lower-case letters to reach seven characters, and then add a period, there are only ~5,000,000,000 choices. That's not many at all -- but the rules look just fine...
1234;lkj rolls off the fingers quite nicely, don't you think?
