invert argument in grep
On 11/9/2006 5:14 AM, Romain Francois wrote:
Hello,
What about an `invert` argument in grep, to return elements that are
*not* matching a regular expression :
R> grep("pink", colors(), invert = TRUE, value = TRUE)
would essentially return the same as :
R> colors() [ - grep("pink", colors()) ]
I'm attaching the files that I modified (against today's tarball) for
that purpose.
I think a more generally useful change would be to be able to return a
logical vector with TRUE for a match and FALSE for a non-match, so a
simple !grep(...) does the inversion. (This is motivated by the recent
R-help discussion of the fact that x[-selection] doesn't always invert
the selection when it's a vector of indices.)
A way to do that without expanding the argument list would be to allow
value="logical"
as well as value=TRUE and value=FALSE.
This would make boolean operations easy, e.g.
colors()[grep("dark", colors(), value="logical")
& !grep("blue", colors(), value="logical")]
to select the colors that contain "dark" but not "blue". (In this case
the RE to select that subset is rather simple because "dark" always
precedes "blue", but if that wasn't true, it would be a lot messier.)
Duncan Murdoch
Cheers,
Romain
------------------------------------------------------------------------
grep <-
function(pattern, x, ignore.case = FALSE, extended = TRUE, perl = FALSE,
value = FALSE, fixed = FALSE, useBytes = FALSE, invert = FALSE)
{
pattern <- as.character(pattern)
## when value = TRUE we return names
if(!is.character(x)) x <- structure(as.character(x), names=names(x))
## behaves like == for NA pattern
if (is.na(pattern)) {
if(value)
return(structure(rep.int(as.character(NA), length(x)),
names = names(x)))
else
return(rep.int(NA, length(x)))
}
if(perl)
.Internal(grep.perl(pattern, x, ignore.case, value, useBytes, invert))
else
.Internal(grep(pattern, x, ignore.case, extended, value, fixed,
useBytes, invert))
}
sub <-
function(pattern, replacement, x, ignore.case = FALSE, extended = TRUE,
perl = FALSE, fixed = FALSE, useBytes = FALSE)
{
pattern <- as.character(pattern)
replacement <- as.character(replacement)
if(!is.character(x)) x <- as.character(x)
if (is.na(pattern))
return(rep.int(as.character(NA), length(x)))
if(perl)
.Internal(sub.perl(pattern, replacement, x, ignore.case, useBytes))
else
.Internal(sub(pattern, replacement, x, ignore.case,
extended, fixed, useBytes))
}
gsub <-
function(pattern, replacement, x, ignore.case = FALSE, extended = TRUE,
perl = FALSE, fixed = FALSE, useBytes = FALSE)
{
pattern <- as.character(pattern)
replacement <- as.character(replacement)
if(!is.character(x)) x <- as.character(x)
if (is.na(pattern))
return(rep.int(as.character(NA), length(x)))
if(perl)
.Internal(gsub.perl(pattern, replacement, x, ignore.case, useBytes))
else
.Internal(gsub(pattern, replacement, x, ignore.case,
extended, fixed, useBytes))
}
regexpr <-
function(pattern, text, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
{
pattern <- as.character(pattern)
text <- as.character(text)
if(perl)
.Internal(regexpr.perl(pattern, text, useBytes))
else
.Internal(regexpr(pattern, text, extended, fixed, useBytes))
}
gregexpr <-
function(pattern, text, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
{
pattern <- as.character(pattern)
text <- as.character(text)
if(perl)
.Internal(gregexpr.perl(pattern, text, useBytes))
else
.Internal(gregexpr(pattern, text, extended, fixed, useBytes))
}
agrep <-
function(pattern, x, ignore.case = FALSE, value = FALSE,
max.distance = 0.1)
{
pattern <- as.character(pattern)
if(!is.character(x)) x <- as.character(x)
## behaves like == for NA pattern
if (is.na(pattern)){
if (value)
return(structure(rep.int(as.character(NA), length(x)),
names = names(x)))
else
return(rep.int(NA, length(x)))
}
if(!is.character(pattern)
|| (length(pattern) < 1)
|| ((n <- nchar(pattern)) == 0))
stop("'pattern' must be a non-empty character string")
if(!is.list(max.distance)) {
if(!is.numeric(max.distance) || (max.distance < 0))
stop("'max.distance' must be non-negative")
if(max.distance < 1) # transform percentages
max.distance <- ceiling(n * max.distance)
max.insertions <- max.deletions <- max.substitutions <-
max.distance
} else {
## partial matching
table <- c("all", "deletions", "insertions", "substitutions")
ind <- pmatch(names(max.distance), table)
if(any(is.na(ind)))
warning("unknown match distance components ignored")
max.distance <- max.distance[!is.na(ind)]
names(max.distance) <- table[ind]
## sanity checks
comps <- unlist(max.distance)
if(!all(is.numeric(comps)) || any(comps < 0))
stop("'max.distance' components must be non-negative")
## extract restrictions
if(is.null(max.distance$all))
max.distance$all <- 0.1
max.insertions <- max.deletions <- max.substitutions <-
max.distance$all
if(!is.null(max.distance$deletions))
max.deletions <- max.distance$deletions
if(!is.null(max.distance$insertions))
max.insertions <- max.distance$insertions
if(!is.null(max.distance$substitutions))
max.substitutions <- max.distance$substitutions
max.distance <- max.distance$all
## transform percentages
if(max.distance < 1)
max.distance <- ceiling(n * max.distance)
if(max.deletions < 1)
max.deletions <- ceiling(n * max.deletions)
if(max.insertions < 1)
max.insertions <- ceiling(n * max.insertions)
if(max.substitutions < 1)
max.substitutions <- ceiling(n * max.substitutions)
}
.Internal(agrep(pattern, x, ignore.case, value, max.distance,
max.deletions, max.insertions, max.substitutions))
}
------------------------------------------------------------------------
\name{grep}
\title{Pattern Matching and Replacement}
\alias{grep}
\alias{sub}
\alias{gsub}
\alias{regexpr}
\alias{gregexpr}
\description{
\code{grep} searches for matches to \code{pattern} (its first
argument) within the character vector \code{x} (second argument).
\code{regexpr} and \code{gregexpr} do too, but return more detail in
a different format.
\code{sub} and \code{gsub} perform replacement of matches determined
by regular expression matching.
}
\usage{
grep(pattern, x, ignore.case = FALSE, extended = TRUE,
perl = FALSE, value = FALSE, fixed = FALSE,
useBytes = FALSE, invert = FALSE)
sub(pattern, replacement, x,
ignore.case = FALSE, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
gsub(pattern, replacement, x,
ignore.case = FALSE, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
regexpr(pattern, text, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
gregexpr(pattern, text, extended = TRUE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
}
\arguments{
\item{pattern}{character string containing a \link{regular expression}
(or character string for \code{fixed = TRUE}) to be matched
in the given character vector. Coerced by
\code{\link{as.character}} to a character string if possible.}
\item{x, text}{a character vector where matches are sought, or an
object which can be coerced by \code{as.character} to a character vector.}
\item{ignore.case}{if \code{FALSE}, the pattern matching is \emph{case
sensitive} and if \code{TRUE}, case is ignored during matching.}
\item{extended}{if \code{TRUE}, extended regular expression matching
is used, and if \code{FALSE} basic regular expressions are used.}
\item{perl}{logical. Should perl-compatible regexps be used?
Has priority over \code{extended}.}
\item{value}{if \code{FALSE}, a vector containing the (\code{integer})
indices of the matches determined by \code{grep} is returned, and if
\code{TRUE}, a vector containing the matching elements themselves is
returned.}
\item{fixed}{logical. If \code{TRUE}, \code{pattern} is a string to be
matched as is. Overrides all conflicting arguments.}
\item{useBytes}{logical. If \code{TRUE} the matching is done
byte-by-byte rather than character-by-character. See Details.}
\item{replacement}{a replacement for matched pattern in \code{sub} and
\code{gsub}. Coerced to character if possible. For \code{fixed =
FALSE} this can include backreferences \code{"\\1"} to
\code{"\\9"} to parenthesized subexpressions of \code{pattern}. For
\code{perl = TRUE} only, it can also contain \code{"\\U"} or
\code{"\\L"} to convert the rest of the replacement to upper or
lower case.
}
\item{invert}{logical. If \code{TRUE}, then the match is reversed
in order to get the items (or the values) that are \emph{not} matching the
pattern}
}
\details{
Arguments which should be character strings or character vectors are
coerced to character if possible.
The two \code{*sub} functions differ only in that \code{sub} replaces
only the first occurrence of a \code{pattern} whereas \code{gsub}
replaces all occurrences.
For \code{regexpr} it is an error for \code{pattern} to be \code{NA},
otherwise \code{NA} is permitted and matches only itself.
The regular expressions used are those specified by POSIX 1003.2,
either extended or basic, depending on the value of the
\code{extended} argument, unless \code{perl = TRUE} when they are
those of PCRE, \url{http://www.pcre.org/}.
(The exact set of patterns supported may depend on the version of
PCRE installed on the system in use, if \R was configured to use the
system PCRE. \R's internal copy used PCRE 6.7.)
\code{useBytes} is only used if \code{fixed = TRUE} or \code{perl = TRUE}.
For \code{grep} its main effect is to avoid errors/warnings about
invalid inputs, but for \code{regexpr} it changes the interpretation
of the output.
}
\value{
For \code{grep} a vector giving either the indices of the elements of
\code{x} that yielded a match or, if \code{value} is \code{TRUE}, the
matched elements of \code{x} (after coercion, preserving names but no
other attributes).
For \code{sub} and \code{gsub} a character vector of the same length
and with the same attributes as \code{x} (after possible coercion).
For \code{regexpr} an integer vector of the same length as \code{text}
giving the starting position of the first match, or \eqn{-1} if there
is none, with attribute \code{"match.length"} giving the length of the
matched text (or \eqn{-1} for no match). In a multi-byte locale these
quantities are in characters rather than bytes unless
\code{useBytes = TRUE} is used with \code{fixed = TRUE} or
\code{perl = TRUE}.
For \code{gregexpr} a list of the same length as \code{text} each
element of which is an integer vector as in \code{regexpr}, except
that the starting positions of every match are given.
If in a multi-byte locale the pattern or replacement is not a valid
sequence of bytes, an error is thrown. An invalid string in \code{x}
or \code{text} is a non-match with a warning for \code{grep} or
\code{regexpr}, but an error for \code{sub} or \code{gsub}.
}
\section{Warning}{
The standard regular-expression code has been reported to be very slow
when applied to extremely long character strings
(tens of thousands of characters or more): the code used when
\code{perl = TRUE} seems much faster and more reliable for such
usages.
The standard version of \code{gsub} does not substitute correctly
repeated word-boundaries (e.g. \code{pattern = "\\b"}).
Use \code{perl = TRUE} for such matches.
The \code{perl = TRUE} option is only implemented for single-byte and
UTF-8 encodings, and will warn if used in a non-UTF-8 multi-byte
locale (unless \code{useBytes = TRUE}).
}
\references{
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole (\code{grep})
}
\seealso{
\link{regular expression} (aka \code{\link{regexp}}) for the details
% the `aka' above is for ESS (and ?reg....) where a space is problematic
of the pattern specification.
\code{\link{glob2rx}} to turn wildcard matches into regular expressions.
\code{\link{agrep}} for approximate matching.
\code{\link{tolower}}, \code{\link{toupper}} and \code{\link{chartr}}
for character translations.
\code{\link{charmatch}}, \code{\link{pmatch}}, \code{\link{match}}.
\code{\link{apropos}} uses regexps and has nice examples.
}
\examples{
grep("[a-z]", letters)
txt <- c("arm","foot","lefroo", "bafoobar")
if(any(i <- grep("foo",txt)))
cat("'foo' appears at least once in\n\t",txt,"\n")
i # 2 and 4
txt[i]
## Double all 'a' or 'b's; "\\" must be escaped, i.e., 'doubled'
%% and escaped even once more in this *.Rd file!
gsub("([ab])", "\\\\1_\\\\1_", "abc and ABC")
txt <- c("The", "licenses", "for", "most", "software", "are",
"designed", "to", "take", "away", "your", "freedom",
"to", "share", "and", "change", "it.",
"", "By", "contrast,", "the", "GNU", "General", "Public", "License",
"is", "intended", "to", "guarantee", "your", "freedom", "to",
"share", "and", "change", "free", "software", "--",
"to", "make", "sure", "the", "software", "is",
"free", "for", "all", "its", "users")
( i <- grep("[gu]", txt) ) # indices
stopifnot( txt[i] == grep("[gu]", txt, value = TRUE) )
## Note that in locales such as en_US this includes B as the
## collation order is aAbBcCdEe ...
(ot <- sub("[b-e]",".", txt))
txt[ot != gsub("[b-e]",".", txt)]#- gsub does "global" substitution
txt[gsub("g","#", txt) !=
gsub("g","#", txt, ignore.case = TRUE)] # the "G" words
regexpr("en", txt)
gregexpr("e", txt)
## trim trailing white space
str = 'Now is the time '
sub(' +$', '', str) ## spaces only
sub('[[:space:]]+$', '', str) ## white space, POSIX-style
sub('\\\\s+$', '', str, perl = TRUE) ## Perl-style white space
## capitalizing
gsub("(\\\\w)(\\\\w*)", "\\\\U\\\\1\\\\L\\\\2", "a test of capitalizing", perl=TRUE)
gsub("\\\\b(\\\\w)", "\\\\U\\\\1", "a test of capitalizing", perl=TRUE)
}
\keyword{character}
\keyword{utilities}
------------------------------------------------------------------------
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