r – convenient way to select columns within a function based on regex within dplyr

I am trying to select columns based on a regex for a function within a dplyr command.

Maybe I am thinking to complicated or I have to reframe my data first?

I provide a reproducible example. I would love to define the columns to use inside length with a regular expression. So in my case col_of_interest are all the columns with a inside…

library(dplyr)

set.seed(1)
col_of_interest <- 'a'
df <- 
  data.frame(
    id=rep(1:2, each=5),
    a1=rnorm(10),
    a2=c(NA, NA, NA, rnorm(7)),
    b1=rnorm(10)
  )

df %>% 
  group_by(id) %>% 
  reframe(value_count=length(na.omit(c(a1, a2))))