replace - Replacing '+' with ', ' for labels in QGIS?

I want to replace a '+' with ', ' to label an attribute looking like "ZT10+ZT20+ZT30" to get "ZT10, ZT20, ZT30" using

regexp_replace("zone_nr", '+', ', ')

but I get an Eval Error: Invalid regular expression '+': quantifier does not follow a repeatable item.

Answer

A + is a special symbol to the QGis regular expression engine (it means 1 or more of the preceding character), so you need to escape it by putting a \ in front of it. So you need:

regexp_replace("zone_nr", '\\+', ', ')