Well, one problem is neither of your regex will match your string; the first will match strings which include 0-9 seven times in succession; the second will match seven consecutive copies of 0-9 or 0- or any combination of these; neither will match 7 random digits.

I'm not sure why you are saying that, but the OP's first pattern WILL match 7 random digits. The second will also match 7 random digits, but it will also match 1-6 random digits as well.

Unfortunately, the OP is a bit unclear as to whether this sequence of 7 numbers should be isolated or part of a larger string.