\Header{grep}{Pattern Matching and Replacement}
\alias{gsub}{grep}
\alias{regexpr}{grep}
\alias{sub}{grep}
\keyword{character}{grep}
\keyword{utilities}{grep}
\begin{Description}\relax
\code{grep} searches for matches to \code{pattern} (its first
argument) within the character vector \code{x} (second
argument). \code{regexpr} does too, but returns more detail in a
different format.

\code{sub} and \code{gsub} perform replacement of matches
determined by regular expression matching.\end{Description}
\begin{Usage}
\begin{verbatim}
grep(pattern, x, ignore.case=FALSE, extended=TRUE, value=FALSE)
sub(pattern, replacement, x,
        ignore.case=FALSE, extended=TRUE)
gsub(pattern, replacement, x,
        ignore.case=FALSE, extended=TRUE)
regexpr(pattern, text,  extended=TRUE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{pattern}] character string containing a regular expression
to be matched in the vector of character string \code{vec}.
\item[\code{x, text}] a character vector where matches are sought.
\item[\code{ignore.case}] if \code{FALSE}, the pattern matching is
\emph{case sensitive} and if \code{TRUE}, case is ignored during matching.
\item[\code{extended}] if \code{TRUE}, extended regular expression matching
is used, and if \code{FALSE} basic regular expressions are used.
\item[\code{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[\code{replacement}] a replacement for matched pattern in
\code{sub} and \code{gsub}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
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.

The regular expressions used are those specified by POSIX 1003.2,
either extended or basic, depending on the value of the
\code{extended} argument.\end{Details}
\begin{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.

For \code{sub} and \code{gsub} a character vector of the same
length as the original.

For \code{regexpr} an integer vector of the same length as
\code{text} giving the starting position of the first match, or -1
if there is none, with attribute \code{"match.length"} giving the
length of the matched text (or -1 for no match).\end{Value}
\begin{SeeAlso}\relax
\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.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
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'
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) )
(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)
\end{ExampleCode}
\end{Examples}

