patterns: Obtain matching indices corresponding to patterns

In data.table: Extension of `data.frame`

Obtain matching indices corresponding to patterns

Description

patterns returns the elements of cols that match the regular expression patterns, which must be supported by grep.

From v1.9.6, melt.data.table has an enhanced functionality in which measure.vars argument can accept a list of column names and melt them into separate columns. See the Efficient reshaping using data.tables vignette linked below to learn more.

Usage

patterns(
  ..., cols=character(0),
  ignore.case=FALSE, perl=FALSE,
  fixed=FALSE, useBytes=FALSE)

Arguments

...	A set of regular expression patterns.
cols	A character vector of names to which each pattern is matched.
ignore.case, perl, fixed, useBytes	Passed to grep.

See Also

melt, https://github.com/Rdatatable/data.table/wiki/Getting-started

Examples

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", cols=names(DT)))
# when used with melt, 'cols' is implicitly assumed to be names of input
# data.table, if not provided.
melt(DT, measure.vars = patterns("^x", "^y"))

data.table documentation built on Aug. 8, 2025, 6:20 p.m.