ah ok, so we are preserving the total number of digits, not the total number of zeros. Gotcha!

No, foobar00999 => foobar01000 there should be 5 digits in both strings

See the last example:

foo099 -> foo100

so foobar00999 => foobar001000, not foobar01000 right?

Attention: If the number has leading zeros the amount of digits should be considered.

You can print the input yourself, but in that case it was foobar00999

i believe there is an issue for one of the test cases here (ruby)

Expected: "foobar01000", instead got: "foobar001000"

if it expected "foobar01000" then the input should be "foobar0999"?
using this as my input does indeed yield "foobar01000"
did you add an extra "0" in this test case?