As you say, the bug is evidently in irregex. It is present both in the "0.9.6: 2016/12/05" version from SchemeBBS and in the latest "0.9.7: 2019/12/31"
Yes I have tested against this version as well and the bug is still there. But at least Alex Shinn is still maintaining irregex. It just needs nice a PR with those reproducible steps.
what did you do with an MBR-partitioned hard disk and a BIOS that refuses to boot MBR?
It came a few weeks after an SSD failure, my system wasn't even fully configured yet. It happened 2 days after I finally installed MIT/GNU Scheme to work locally on SchemeBBS. I wiped the disk and quickly reinstalled linux, a couple of dotfiles, sent the faulty battery pack back, and I'm back to where I was before the incident. Relaunching SchemeBBS from the mobile phone wasn't really fun. I felt I didn't have time to instigate how one can boot from MBR on an UEFI locked BIOS. I had no documents to lose, so...
My laptop is falling apart, I'm looking for a used Thinkpad. There are some good deals and they're rock solid. The perfect machine for someone like me.
Give me another few days, kind anon, and I'll write a full documentation for the SchemeBBS installation. I have to do it myself, and I will take notes along the way. I'll share the nginx.conf file too, I don't think it's too much of a security issue.