You can do that with regular expressions .
It can be done in base R or e.g. with the nice package stringr
columnA <-c("123.456.789", "12.345.678", "12..678", "ABC50BC4304")
pattern <- "^\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}"
grepl(pattern,columnA)
#> [1] TRUE TRUE FALSE FALSE
library(stringr)
stringr::str_detect(columnA,pattern)
#> [1] TRUE TRUE FALSE FALSE
Created on 2021-08-02 by the reprex package (v2.0.0)