Add leading zeros only if the number starts with 6 in R dataframe

I have a dataframe with numbers that I need to format. How do I add leading zeroes to only numbers that starts with 6? All examples seen using str_pad() or sprintf() are not exactly like my task, and I found it challenging to adapt them. My dummy dataframe is below:

dummy_numbers
621103 
06102658  
19562106    
61102
0635467

The desired result is:

desired_numbers
0621103 
06102658  
19562106    
061102
0635467

Thanks.

>Solution :

Your can use grepl() and regex (^) to capture the start of a string.

library(tidyverse)

df %>% mutate(dummy_numbers = ifelse(grepl("^6", dummy_numbers), 
                                     paste0(0, dummy_numbers), 
                                     dummy_numbers))