The difficulty you face is in deciding which of the commas in

  concat(int2string(counter,1),result)
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is the one that separates the arguments. In the trivial case of two strings, this seems easy, except that the strings themselves might contain commas. But when you introduce arbitrary expressions into the mix, you're past the point where you can deal with this problem safely using a regular expression, and into territory where you need a parser to correctly distinguish the two arguments.

Can you say a bit more about the types of expressions that might occur as arguments to concat()? If there's a limited set, you can special-case them with one regular expression per combination.