Column selection without `with = FALSE` is dependent on J expression

Start by reproducing the two iris data.table expressions from the issue and trace how the j expression handles c, paste0, and grep. Done means the column-selection behavior is consistent with the documented syntax, with regression coverage for both examples and the intended with = FALSE or parenthesized alternatives.

(This question is asked on SO with some helpful comments)

Consider the following two examples. The J expresssion is similar in option 1 and option 2, but the results are different: option 1 results in column selection while option 2 is evaluated into a character vector.

> library(data.table)
> iris <- as.data.table(iris)

> # option 1
> iris[, c('Species', paste0(c('Sepal.', 'Petal.'), 'Length'))]
       Species Sepal.Length Petal.Length
  1:    setosa          5.1          1.4
  2:    setosa          4.9          1.4
  3:    setosa          4.7          1.3
  4:    setosa          4.6          1.5
  5:    setosa          5.0          1.4
 ---                                    
146: virginica          6.7          5.2
147: virginica          6.3          5.0
148: virginica          6.5          5.2
149: virginica          6.2          5.4
150: virginica          5.9          5.1

> # option 2
> iris[, c('Species', grep('Length', names(iris), value = TRUE))] 
[1] "Species"      "Sepal.Length" "Petal.Length"

In the comment of the SO post, David Arenburg found that "functions such paste and c are being explicitly searched in the j expression and then set with=FALSE, similar to what happens with .. prefix. grep isn't being searched". From a user's perspective, I think this is very unconvenient. Rather than gusssing what users want to do, it is easier to explicit require users specifiy their needs using with = FALSE or other syntax such as (c(...)) as commented by G. Grothendieck on the SO post.

Here is the sessionInfo:

> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.3 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/atlas/libblas.so.3.10.3
LAPACK: /usr/lib/x86_64-linux-gnu/atlas/liblapack.so.3.10.3

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.12.2

loaded via a namespace (and not attached):
[1] compiler_3.6.1 tools_3.6.1