The regex + modifier doesn't work at all properly with non-capture groups that happen to end on another modifier:
mob
Login()
..()
var/regex/rgx = new/regex("((?:.+)+")
if(rgx.Find("something"))
world << "Regex isn't broken"
else
world << "Regex is broken"
https://regex101.com/r/0aSWZZ/5/
As you can see, the above regex should match "something".
But when you run it in BYOND, you get the error message indicating that regex is broken.