regex – Dataweave how to use variables in regular expression

I want to use a regular expression in Mulesoft Dataweave and substitute some values by using variables.

I have this code which excecutes fine in Dataweave:

"1000000087" replace /^(.{3}).(.{6})$/ with ($[1] ++ "H" ++ $[2])

//output

"100H000087"

I want to substitute value 3 and 6 for the number of digits with variables.
I have tried this:

var prefix = 3
var suffix = 6

"1000000087" replace /^(.{prefix}).(.{suffix})$/ with ($[1] ++ "H" ++ $[2])

This gives this error:

invalid Regex: Illegal repetition near index 2
^(.{prefix}).(.{suffix})$
  ^

26| "1000000087" replace /^(.{prefix}).(.{suffix})$/ with ($[1] ++ "H" ++ $[2])
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Location:
main (line: 26, column:22)

How to solve this ?