Need help with a mask (Brute force, but, I know a lot about the hash)

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This is not supported by default. Best what you can do is to write a program that permutates the variables and handle the seperation. If you want to use oclHashcat-plus as an accelertor, you can using its stdin interface but notice for fast hashes like MD5 you have to generate enough work for it to run efficiently. that could be, for example, brute force the C part using rules and insert it at an appropritate position. However, this is advanced stuff, it will not work without a bit of writing code by yourself.