How to delete text that does not start with a certain amount of numbers in dataframe column

I have this:

col1
1234HO
9535KU
4532SP
1
hello
xyz
1206
9530OK
23
8524US

And I need it to be this:

col1     col2    col3
1234HO   1234    HO
9535KU   9535    KU
4532SP   4532    SP
                       #these rows still need to be there


1206     1206          #keep in mind that I still want to keep this if there is 4 numbers
9530OK   9530    OK

8524US   8524    US

I tried removing it manually, but it's a bit too much work. I am not sure how to make a function that says "delete all text that does not start with 4 numbers". I would only know how to do it if they were all the same numbers, but they can be any numbers.

Solution

Here is a dplyr approach with very low level of regex.

Input

# your df
df <- read.table(header = T, text = "
col1
1234HO
9535KU
4532SP
1
hello
xyz
1206
9530OK
23
8524US")

Empty rows

library(dplyr)

df %>% mutate(col2 = str_extract(col1, "^[0-9]{4,}"), 
              col3 = str_extract(col1, "[A-Z].*$"),
              col3 = replace_na(col3, ""),
              across(everything(), ~ifelse(grepl("^[0-9]{4}", col1), .x, print(""))))

     col1 col2 col3
1  1234HO 1234   HO
2  9535KU 9535   KU
3  4532SP 4532   SP
4                  
5                  
6                  
7    1206 1206     
8  9530OK 9530   OK
9                  
10 8524US 8524   US

Rows as NA

# if you want them to be filled with NA
df %>% mutate(col2 = str_extract(col1, "^[0-9]{4,}"), 
              col3 = str_extract(col1, "[A-Z].*$"),
              across(everything(), ~ifelse(grepl("^[0-9]{4}", col1), .x, NA)))

     col1 col2 col3
1  1234HO 1234   HO
2  9535KU 9535   KU
3  4532SP 4532   SP
4    <NA> <NA> <NA>
5    <NA> <NA> <NA>
6    <NA> <NA> <NA>
7    1206 1206 <NA>
8  9530OK 9530   OK
9    <NA> <NA> <NA>
10 8524US 8524   US