Nice! I tried a spin-off based on binary comprehension out of curiosity, but it couldn’t beat recursion in the benchmarks (link) 
length = byte_size(str)
offset = rem(length - 1, 3) + 1
{acc, rest} = String.split_at(str, offset)
for <<group::binary-3 <- rest>>, into: acc, do: <<"_", group::binary-3>>
Results:
##### With input 7 digits #####
Name ips average deviation median 99th %
Comprehension 3.95 M 253.43 ns ±12903.32% 167 ns 292 ns
Recursive 3.70 M 270.12 ns ±12392.13% 166 ns 292 ns
Comparison:
Comprehension 3.95 M
Recursive 3.70 M - 1.07x slower +16.69 ns
Memory usage statistics:
Name Memory usage
Comprehension 680 B
Recursive 488 B - 0.72x memory usage -192 B
##### With input 30 digits #####
Name ips average deviation median 99th %
Recursive 3.00 M 333.77 ns ±3331.98% 292 ns 458 ns
Comprehension 1.91 M 522.30 ns ±3697.19% 459 ns 584 ns
Comparison:
Recursive 3.00 M
Comprehension 1.91 M - 1.56x slower +188.53 ns
Memory usage statistics:
Name Memory usage
Recursive 0.95 KB
Comprehension 1.95 KB - 2.06x memory usage +1 KB
