closes #3319 -- manually escape backslash in character by processing

NEWS.md

@@ -14,6 +14,8 @@
 
 3. Default print output (top 5 and bottom 5 rows) when ncol>255 could display the columns in the wrong order, [#3306](https://github.com/Rdatatable/data.table/issues/3306). Thanks to Kun Ren for reporting.
 
+4. Grouping `by = 'string_with_\\'` would fail, [#3319](https://github.com/Rdatatable/data.table/issues/3319). Thanks to @HughParsonage for reporting and @MichaelChirico for the fix.
+
 #### NOTES
 
 1. When upgrading to 1.12.0 some Windows users might have seen `CdllVersion not found` in some circumstances. We found a way to catch that so the [helpful message](https://twitter.com/MattDowle/status/1084528873549705217) now occurs for those upgrading from versions prior to 1.12.0 too, as well as those upgrading from 1.12.0 to a later version. See item 1 in notes section of 1.12.0 below for more background.

R/data.table.R

@@ -866,8 +866,10 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
             if (length(bysub)>1L) stop("'by' is a character vector length ",length(bysub)," but one or more items include a comma. Either pass a vector of column names (which can contain spaces, but no commas), or pass a vector length 1 containing comma separated column names. See ?data.table for other possibilities.")
             bysub = strsplit(bysub,split=",")[[1L]]
           }
-          tt = grep("^[^`]+$",bysub)
-          if (length(tt)) bysub[tt] = paste0("`",bysub[tt],"`")
+          backtick_idx = grep("^[^`]+$",bysub)
+          if (length(backtick_idx)) bysub[backtick_idx] = paste0("`",bysub[backtick_idx],"`")
+          backslash_idx = grep("\\", bysub, fixed = TRUE)
+          if (length(backslash_idx)) bysub[backslash_idx] = gsub('\\', '\\\\', bysub[backslash_idx], fixed = TRUE)
           bysub = parse(text=paste0("list(",paste(bysub,collapse=","),")"))[[1L]]
           bysubl = as.list.default(bysub)
         }

inst/tests/tests.Rraw

@@ -13320,6 +13320,12 @@ test(1982.1, tt[1L], "")
 tt = as.integer(tt[tt!=""])
 test(1982.2, tt, seq_along(tt))
 
+# parse(text = 'list(`\\phantom{.}`)') fails, #3319
+library(data.table)
+DT <- data.table(x = 1, y = 1:5)
+setnames(DT, "x", "\\phantom{.}")
+test(1983.1, DT[, .(y = mean(y)), keyby = "\\phantom{.}"],
+     data.table(`\\phantom{.}` = 1, y = 3, key = '\\phantom{.}'))
 
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 #  Add new tests above this line  #