Closes #1294. Added patterns.Rd

README.md

@@ -45,6 +45,7 @@
   11. `melt.data.table` 
       * can now melt into multiple columns by providing a list of columns to `measure.vars` argument. Closes [#828](https://github.com/Rdatatable/data.table/issues/828). Thanks to Ananda Mahto for the extended email discussions and ideas on generating the `variable` column.
       * also retains attributes wherever possible. Closes [#702](https://github.com/Rdatatable/data.table/issues/702) and [#993](https://github.com/Rdatatable/data.table/issues/993). Thanks to @richierocks for the report.
+      * Added `patterns.Rd`. Closes [#1294](https://github.com/Rdatatable/data.table/issues/1294). Thanks to @MichaelChirico.
 
   12. `.SDcols`
       * understands `!` now, i.e., `DT[, .SD, .SDcols=!"a"]` now works, and is equivalent to `DT[, .SD, .SDcols = -c("a")]`. Closes [#1066](https://github.com/Rdatatable/data.table/issues/1066). 

man/patterns.Rd

@@ -0,0 +1,24 @@
+\name{patterns}
+\alias{patterns}
+\title{Regex patterns to extract columns from data.table}
+\description{
+  From \code{v1.9.6}, \code{\link{melt.data.table}} has a new enhanced functionality in which \code{measure.vars} argument can accept a \emph{list of column names} and melt them into separate columns. See the \code{Efficient reshaping using data.tables} vignette linked below to learn more.
+
+  \code{patterns} is designed purely for convenience, to be used only within the \code{measure.vars} argument of \code{melt.data.table}. Column names corresponding to each pattern from the \code{data.table} is melted into a separate column.
+}
+\usage{
+patterns(...)
+}
+\arguments{
+  \item{...}{ A set of patterns. See example. }
+}
+\seealso{ 
+  \code{\link{melt}}, \url{https://github.com/Rdatatable/data.table/wiki/Getting-started}
+}
+\examples{
+# makes sense only in the context of melt at the moment
+dt = data.table(x1 = 1:5, x2 = 6:10, y1 = letters[1:5], y2 = letters[6:10])
+# melt all columns that begin with 'x' & 'y', respectively, into separate columns
+melt(dt, measure.vars = patterns("^x", "^y"))
+}
+\keyword{ data }