String column dataframe: replace for another string built from a substring from each row

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First, please update DataFrames.jl to 0.22 release to get the latest and bug free functionality.

Now regarding your code the simple transformation is (it assumes that DRE contains ONLY ASCII digits):

transform!(df, :DRE => ByRow(x -> "20"*x[2:3]) => :DRE)

or a general solution that correctly handles all UNICODE characters but is a bit verbose:

transform!(df, :DRE => ByRow(x -> "20"*last(first(x, 3), 2)) => :DRE)
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