Copy-paste files to folders that have matching names using R

I am trying to copy files to various folders that have matching filenames.

Here's an extract of the filenames:

20201026_ABCD.txt
20201026_XYZ.txt
20201027_ABCD.txt
20201027_POR.txt
20201028_ABCD.txt
20201028_PQR.txt

I want to create folders that have just the date components from the files above. I have managed to get that far based on the code below:

setwd("C:/Projects/TEST")
        
library(stringr)
        
filenames<-list.files(path = "C:/Projects/TEST", pattern = NULL)
        
#create a variable that contains all the desired filenames
foldernames.unique<-unique(str_extract(filenames,"[0-9]{1,8}"))
    
#create folders based on this variable
foldernames.unique<-paste("dates/",foldernames.unique,sep='')
lapply(foldernames.unique,dir.create,recursive = TRUE)

Now, how do I copy 20201026_ABCD.txt and 20201026_XYZ.txt to the folder 20201026, so on and so forth?

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