in reply to Converting To Uuencoded Format

how does the following do that in one step?

It's a lookup table.

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_

The tr/// pattern condenses the table into ranges.
The description you gave condenses the table into a math formula.