How to change character to numeric vector in R

I'm guessing that this is from a JSON source, so there are packages for that:

 library(jsonlite) # obviously needs to be installed first

fromJSON(a)
#[1] 1 2 3

JSON files always are read into R functions from type character but the converted to R objects with typing conventions like that of read.table.

Clean the string with gsub, split it on comma and convert it to numeric.

b <- as.numeric(unlist(strsplit(gsub('\\[|\\]', '', a), ',\\s+')))
b
#[1] 1 2 3 

I prefer to do a regex find all to extract all digits. Then, cast the character vector output to numeric:

a <- "[1, 2, 3]"
b <- as.numeric(regmatches(a, gregexpr("[0-9]+", a))[[1]])
b

[1] 1 2 3

stringr (tidyverse) way of doing it

library(stringr)
a <- "[1, 2, 3]"

str_split(a, ',') %>% unlist %>%
  str_replace('\\D*(\\d*)\\D*', '\\1') %>%
  as.numeric()
#> [1] 1 2 3

^{Created on 2021-06-27 by the reprex package (v2.0.0)}