R: Only keep the 3 (x) first characters in a all rows in a column?

Question

I have imported a few thousand xls files into a data.frame and I added a column with the filename.

Thus I have the data

data1  data2  data3  filname
A          


        

Answer 1

df$filname <- sub("^(\\d{3}).*$", "\\1", df$filname)

or

df$filname <- substr(df$filname, 0, 3)