I thought that this would be a case of the Tilde being a special character for your target document due to its common use in HL7 for repeating fields. However, I ran a test to see what I got when trying this.

I created a transform for a PV1 segment, and attempted to set the value of PV1:1 to the output of the replace function and the input string contained a few commas:

I then ran this, and got this result:

Not only did it successfully replace the commas with tildes, but the virtual document now see's it as a repeating segment (even though the field is not repeating in it's specification).

I know this doesn't directly help you, but wanted to share my results in case it helped lead you to finding a solution. (for ref, this is from version 2022.1 of Iris For Health)

Ahh, I see what you mean.

I have not had to work in this way with IRIS and OpenEHR, so unfortunately I wouldn't be able to provide much insight.

Hey Joost.

What do you mean when you say "handle"?

Our interactions with OpenEHR for reading and creating/updating compositions are generally via traditional HTTP based API's, which is relatively simple to set up with IRIS.