• I followed the official installation instructions
• I reproduced the error with the newest versions of IRkernel, IRdisplay, and repr
• I included a minimal reproducible example
• This the right repository: I think the issue is with IRkernel, not
  IRkernel/repr or a third party repository

So that's hard to reproduce, but I'm getting this a lot in JupyterLab with LSP extension installed. The LSP extension can trigger completions after typing any character, so it may sometimes trigger in odd places and I think this leads to some edge case in completions handling; pretty often the kernel will die and restart with the following message in the console:

Error in suppressWarnings(gregexpr("[^\\.\\w:?$@[\\]]+", linebuffer, perl = TRUE))[[1L]] :
  subscript out of bounds
Calls: <Anonymous> ... handle_shell -> complete -> completions -> <Anonymous>
Execution halted
[I 2021-12-01 15:50:58.092 ServerApp] AsyncIOLoopKernelRestarter: restarting kernel (1/5), keep random ports
kernel 6940fbfd-10d1-4918-ada7-7028d880a2f3 restarted

It seems to come from this call:

To a core function which was not modified in the last 8 years:

https://github.com/wch/r-source/blob/79298c499218846d14500255efd622b5021c10ec/src/library/utils/R/completion.R#L286-L291

I wonder, is this off-by-one error on my side, or potentially something in IRKernel? In either case I thought it would be worth to record an issue here to ask if anyone saw something similar.

I wonder if this could be that sometimes completing on empty line triggers this issue?