grep(pattern, x, ignore.case = FALSE, perl = FALSE, value = FALSE, fixed = FALSE, useBytes = FALSE, invert = FALSE)
• pattern: string to be matched, supports regular expression
• x: string or string vector
• ignore.case: if FALSE, the pattern matching is case sensitive and if TRUE, case is ignored during matching
• perl: logical. Should perl-compatible regexps be used? Has priority over extended
• fixed: logical. If TRUE, pattern is a string to be matched as is. Overrides all conflicting arguments
• useBytes: logical. If TRUE the matching is done byte-by-byte rather than character-by-character
• invert: logical. If TRUE return indices or values for elements that do not match
grep(value = FALSE) returns an integer vector of the indices
of the elements of x that yielded a match (or not, for
invert = TRUE).
> grep("rect", "draw a rectangle")
[1] 1
> str <- c("Regular", "expression", "examples of R language")
> x <- grep("ex",str,value=F)
> x
[1] 2 3
> x <- "line 4322: He is now 25 years old, and weights 130lbs";
> x <- grep("\\d","",x)
> x
[1] 1
• grep(value = TRUE) returns a character vector containing the
selected elements of x (after coercion, preserving names but no
other attributes).
> grep("rect", "draw a rectangle", value=T)
[1] "draw a rectangle"
> x <- grep("ex",str,value=T)
> x
[1] "expression" "examples of R language"
• grepl returns a logical vector (match or not for each element of
x).
> x <- grepl("ex",str)
> x
[1] FALSE TRUE TRUE
R has various functions for regular expression based match and replaces. The grep, grepl, regexpr and gregexpr functions are used for searching
for matches, while sub and gsub for performing replacement.
• sub and gsub return a character vector of the same
length and with the same attributes as x (after possible
coercion to character). Elements of character vectors x which
are not substituted will be returned unchanged (including any declared
encoding). If 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 fixed = TRUE).
> str <- c("Regular", "expression", "examples of R language")
> x <- sub("x.ress","",str)
> x
[1] "Regular" "eion" "examples of R language"
> x <- sub("x.+e","",str)
> x
[1] "Regular" "ession" "e"
> x <- "line 4322: He is now 25 years old, and weights 130lbs";
> x <- gsub("[[:digit:]]","",x)
> x
[1] "line : He is now years old, and weights lbs"
> x <- "line 4322: He is now 25 years old, and weights 130lbs";
> x <- gsub("\\d+","",x)
> x
[1] "line : He is now years old, and weights lbs"
• regexpr returns an integer vector of the same length as
text giving the starting position of the first match or
-1 if there is none, with attribute "match.length", an
integer vector giving the length of the matched text (or -1 for
no match). The match positions and lengths are in characters unless
useBytes = TRUE is used, when they are in bytes.
> str <- c("Regular", "expression", "examples of R language")
> x <- regexpr("x*ress",str)
> x
[1] -1 4 -1
• gregexpr returns a list of the same length as text each
element of which is of the same form as the return value for regexpr,
except that the starting positions of every (disjoint) match are
given.
> str <- c("Regular", "expression", "examples of R language")
> x <- gregexpr("x*ress",str)
> x
[[1]] [1] -1 attr(,"match.length") [1] -1 attr(,"useBytes") [1] TRUE [[2]] [1] 4 attr(,"match.length") [1] 4 attr(,"useBytes") [1] TRUE [[3]] [1] -1 attr(,"match.length") [1] -1 attr(,"useBytes") [1] TRUE
Regular Expression Syntax:
