% File src/library/base/man/grep.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2009 R Core Development Team
% Distributed under GPL 2 or later

\name{grep}
\title{Pattern Matching and Replacement}
\alias{grep}
\alias{grepl}
\alias{sub}
\alias{gsub}
\alias{regexpr}
\alias{gregexpr}
\description{
  \code{grep}, \code{grepl}, \code{regexpr} and \code{gregexpr} search
  for matches to argument \code{pattern} within each element of a
  character vector: they differ in the format of and amount of detail in
  the results.

  \code{sub} and \code{gsub} perform replacement of the first and all
  matches respectively.
}
\usage{
grep(pattern, x, ignore.case = FALSE, perl = FALSE, value = FALSE,
     fixed = FALSE, useBytes = FALSE, invert = FALSE)

grepl(pattern, x, ignore.case = FALSE, perl = FALSE, fixed = FALSE,
      useBytes = FALSE)

sub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
    fixed = FALSE, useBytes = FALSE)

gsub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
     fixed = FALSE, useBytes = FALSE)

regexpr(pattern, text, ignore.case = FALSE, perl = FALSE,
        fixed = FALSE, useBytes = FALSE)

gregexpr(pattern, text, ignore.case = FALSE, 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.  If a
    character vector of length 2 or more is supplied, the first element
    is used with a warning.  Missing values are allowed except for
    \code{regexpr} and \code{gregexpr}.}
  \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{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
    \sQuote{Details}.}
  \item{invert}{logical.  If \code{TRUE} return indices or values for
         elements that do \emph{not} match.}
  \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 and \code{"\\E"} to end case conversion.  If a
    character vector of length 2 or more is supplied, the first element
    is used with a warning.  If \code{NA}, all elements in the result
    corresponding to matches will be set to \code{NA}.
  }
}
\details{
  Arguments which should be character strings or character vectors are
  coerced to character if possible.

  Each of these functions operates in one of three modes:
  \enumerate{
    \item \code{fixed = TRUE}: use exact matching.
    \item \code{perl = TRUE}: use Perl-style regular expressions.
    \item \code{fixed = FALSE, perl = FALSE}: use POSIX 1003.2
    extended regular expressions.
  }
  See the help pages on \link{regular expression} for details of the
  different types of regular expressions.

  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.  If \code{replacement} contains
  backreferences which are not defined in \code{pattern} the result is
  undefined (but most often the backreference is taken to be \code{""}).

  For \code{regexpr} and \code{gregexpr} it is an error for
  \code{pattern} to be \code{NA}, otherwise \code{NA} is permitted and
  gives an \code{NA} match.

  The main effect of \code{useBytes} is to avoid errors/warnings about
  invalid inputs and spurious matches in multibyte locales, but for
  \code{regexpr} it changes the interpretation of the output.  As from
  \R 2.10.0 it inhibits the conversion of inputs with marked encodings.
  
  Caseless matching does not make much sense for bytes in a multibyte
  locale, and you should expect it only to work for ASCII characters if
  \code{useBytes = TRUE}.
}

\note{
  Prior to \R 2.11.0 there was an argument \code{extended} which could
  be used to select \sQuote{basic} regular expressions: this was often
  used when \code{fixed = TRUE} would be preferable.  In the actual
  implementation (as distinct from the POSIX standard) the only
  difference was that \samp{?}, \samp{+}, \samp{\{}, \samp{|}, \samp{(},
  and \samp{)} were not interpreted as metacharacters.
}

\value{
  \code{grep(value = FALSE)} returns an integer vector of the indices
  of the elements of \code{x} that yielded a match (or not, for
  \code{invert = TRUE}.
  
  \code{grep(value = TRUE)} returns a character vector containing the
  selected elements of \code{x} (after coercion, preserving names but no
  other attributes).

  \code{grepl} returns a logical vector (match or not for each element of
  \code{x}).

  For \code{sub} and \code{gsub} return a character vector of the same
  length and with the same attributes as \code{x} (after possible
  coercion to character).  Elements of character vectors \code{x} which
  are not substituted will be returned unchanged (including any declared
  encoding).  If \code{useBytes = FALSE} a non-ASCII substituted result
  will often be in UTF-8 with a marked encoding (e.g. if there is a
  UTF-8 input, and in a multibyte locale unless \code{fixed = TRUE}).
  Such strings can be re-encoded by \code{\link{enc2native}}.

  \code{regexpr} returns 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"}, an
  integer vector giving the length of the matched text (or \eqn{-1} for
  no match).  The match positions and lengths are in characters unless
  \code{useBytes = TRUE} is used, when they are in bytes.

  \code{gregexpr} returns a list of the same length as \code{text} each
  element of which is of the same form as the return value for \code{regexpr},
  except that the starting positions of every (disjoint) match are
  given.
}

\section{Warning}{
  POSIX 1003.2 mode of \code{gsub} and \code{gregexpr} does not
  work correctly with repeated word-boundaries (e.g. \code{pattern =
  "\\b"}).  Use \code{perl = TRUE} for such matches (but that may not
  work as expected with non-ASCII inputs, as the meaning of
  \sQuote{word} is system-dependent).
}

\section{Performance considerations}{
  If you are doing a lot of regular expression matching, including on
  very long strings, you will want to consider the options used.
  Generally PCRE will be faster than the default regular expression
  engine, and \code{fixed = TRUE} faster still (especially when each
  pattern is matched only a few times).

  If you are working in a single-byte locale and have marked UTF-8
  strings that are representable in that locale, convert them first as
  just one UTF-8 string will force all the matching to be done in
  Unicode, which attracts a penalty of around \eqn{3\times{}}{3x} for
  the default POSIX 1003.2 mode.
  
  If you can make use of \code{useBytes = TRUE}, the strings will not be
  checked before matching, and the actual matching will be faster.
  Often byte-based matching suffices in a UTF-8 locale since byte
  patterns of one character never match part of another.
}

\source{
  The C code for POSIX-style regular expression matching has changed
  over the years. As from \R 2.10.0 the TRE library of Ville Laurikari
  (\url{http://laurikari.net/tre/}) is used.  From 2005 to \R 2.9.2,
  code based on \code{glibc} was used (and before that, code from GNU
  \command{grep}).  The POSIX standard does give some room for
  interpretation, especially in the handling of invalid regular
  expressions and the collation of character ranges, so the results will
  have changed slightly.

  For Perl-style matching PCRE (\url{http://www.pcre.org}) is used.
}

\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole (\code{grep})
}

% the `aka' below is for ESS
\seealso{
  \link{regular expression} (aka \code{\link{regexp}}) for the details
  of the pattern specification.

  \code{\link{glob2rx}} to turn wildcard matches into regular expressions.

  \code{\link{agrep}} for approximate matching.
  
  \code{\link{charmatch}}, \code{\link{pmatch}} for partial matching,
  \code{\link{match}} for matching to whole strings.

  \code{\link{tolower}}, \code{\link{toupper}} and \code{\link{chartr}}
  for character translations.
  
  \code{\link{apropos}} uses regexps and has more examples.
}
\examples{
grep("[a-z]", letters)

txt <- c("arm","foot","lefroo", "bafoobar")
if(length(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'
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
txt <- "a test of capitalizing"
gsub("(\\\\w)(\\\\w*)", "\\\\U\\\\1\\\\L\\\\2", txt, perl=TRUE)
gsub("\\\\b(\\\\w)",    "\\\\U\\\\1",       txt, perl=TRUE)

txt2 <- "useRs may fly into JFK or laGuardia"
gsub("(\\\\w)(\\\\w*)(\\\\w)", "\\\\U\\\\1\\\\E\\\\2\\\\U\\\\3", txt2, perl=TRUE)
 sub("(\\\\w)(\\\\w*)(\\\\w)", "\\\\U\\\\1\\\\E\\\\2\\\\U\\\\3", txt2, perl=TRUE)
}
\keyword{character}
\keyword{utilities}
