[nyberg]

- Passing the allocator gives the user of the container control over where memory is allocated and the scheme used to clean it all up. You can always set a global allocator if you wish, nothing forces you to use it as a parameter.

- Regex can be provided as a library and specialized at comptime. There's never been a need for regex to be part of a primitive type and many other languages do fine without it. Concatenation is a question of "where do you allocate memory" as you have explicit allocation it's not clear what zig should do with it thus the default is to let you call std.mem.concat and dealing with the possible failure or having a look at the problem and asking if you really need concat in the first place. Often you don't as formatting string is better done with std.fmt and ArrayList(u8)'s writer().