Probability of words appearing at specific area in string I was linked a youtube video where the first two characters were underscores, how could I calculate the probability of that happening? 64 possible characters, 11 character long string, and only two underscores appearing twice at the beginning. Is this possible?

If you assume that all characters are equally likely to appear at any position (not likely for any "real" language, but probably a reasonable assumption for auto-generated URLs), then the probability for an underscore at any particular position is $\frac{1}{64}$.
If you also assume that characters are independent (again see above), then the probability for two underscores is $\frac{1}{64}\times\frac{1}{64}\approx 0.0002$.
