Filter concats without macro detour?

[twMat]

I wish concatenations were possible directly in the filter rather having to detour via a macro. The following recurrence is annoying and makes code harder to read:

\define concat() $(foo)$bar

...filter="...prefix<concat>..."

Instead something like the following would be neat and very legible:

...filter="...prefix[<foo>bar]..."

[pmario]

...The following recurrence is annoying and makes code harder to read:

but much more flexible in the long run

[pmario]

BUT ... yes ... concatenation should be easier!

[saqimtiaz]

See #5121 and also some 5.2.0 shenanigans

The challenge is making text substitution easier without adding too much cognitive overload for users in terms of new syntax.