See String#next in ruby.

Ruby document doesn't define the algorithm in String#next clearly. Here is my version (may not be 100% same as ruby).

Alphanumeric runes are following.

• a - z
• A - Z
• 0 - 9

If there is one alphanumeric rune is found in string, increase the rune by 1. If increment generates a "carry", the rune to the left of it is incremented. This process repeats until there is no carry, adding an additional rune if necessary.

If there is no alphanumeric rune, the rightmost rune will be increased by 1 regardless whether the result is a valid rune or not.