#15 Ability to lock certain characters in the password

It would be good to have a feature which enables the user to specify the known and unknown parts of a password. For instance, if i know that i have to crack an 8 digit password, and if i know the last 3 characters of the password, but need to guess the first 5, it should be possible to tell pdfcrack exactly this. I don't know how many people will benefit from this feature, but I ended up needing something like this, and couldn't find it in any software. If we have limited info about a password, why not introduce a feature that can make use of it!?