The modern equivalent of `expr 1 + 1` is $(( 1 + 1 )), but that is not supported by the shell parser embedded in bitbake. Adding support for it would be useful.
Just tested, still doesn't parse.
I'll try to have a look at this one over the next week. I am in the area having just worked on the python code snippets so should be able to slide over to this without a big context switch. I will grab the defect after a quick poke around to ensure I am not biting off something I can't chew.
I have looked into this recently. It is fairly straightforward to get basic cases supported but to implement this in full and correctly it is going to take time. The combination of this being a multi-character delimiter, support for nesting and the '$' being allowed to be dropped inside this compound command all contribute to this being non-trivial. I do intend to continue to plug away at this, so stay tuned.