| grep {base} | R Documentation |
grep searches for matches to pattern (its first
argument) within the character vector x (second argument).
regexpr and gregexpr do too, but return more detail in
a different format.
sub and gsub perform replacement of matches determined
by regular expression matching.
grep(pattern, x, ignore.case = FALSE, extended = TRUE, perl = FALSE,
value = FALSE, fixed = FALSE, useBytes = 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)
pattern |
character string containing a regular expression
(or character string for fixed = TRUE) to be matched
in the given character vector. Coerced to character if possible. |
x, text |
a character vector where matches are sought. Coerced to character if possible. |
ignore.case |
if FALSE, the pattern matching is case
sensitive and if TRUE, case is ignored during matching. |
extended |
if TRUE, extended regular expression matching
is used, and if FALSE basic regular expressions are used. |
perl |
logical. Should perl-compatible regexps be used?
Has priority over extended. |
value |
if FALSE, a vector containing the (integer)
indices of the matches determined by grep is returned, and if
TRUE, a vector containing the matching elements themselves is
returned. |
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. See Details. |
replacement |
a replacement for matched pattern in sub and
gsub. Coerced to character if possible. This can include
backreferences "\1" to "\9" to parenthesized
subexpressions of pattern. For perl = TRUE only, it
can also contain "\U" or "\L" to convert the rest of
the replacement to upper or lower case.
|
Arguments which should be character strings or character vectors are coerced to character if possible.
The two *sub functions differ only in that sub replaces
only the first occurrence of a pattern whereas gsub
replaces all occurrences.
For regexpr it is an error for pattern to be NA,
otherwise 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
extended argument, unless perl = TRUE when they are
those of PCRE,
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.
(The exact set of patterns supported may depend on the version of
PCRE installed on the system in use.)
useBytes is only used if fixed = TRUE or perl = TRUE.
For grep its main effect is to avoid errors/warnings about
invalid inputs, but for regexpr it changes the interpretation
of the output.
For grep a vector giving either the indices of the elements of
x that yielded a match or, if value is TRUE, the
matched elements.
For sub and gsub a character vector of the same length
as the original.
For regexpr 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" giving the length of the
matched text (or -1 for no match). In a multi-byte locale these
quantities are in characters rather than bytes unless
useBytes = TRUE is used with fixed = TRUE or
perl = TRUE.
For gregexpr a list of the same length as text each
element of which is an integer vector as in 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 x
or text is a non-match with a warning for grep or
regexpr, but an error for sub or gsub.
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
perl = TRUE seems much faster and more reliable for such
usages.
The standard version of gsub does not substitute correctly
repeated word-boundaries (e.g. pattern = "\b").
Use perl = TRUE for such matches.
The 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 useBytes = FALSE).
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
The New S Language.
Wadsworth & Brooks/Cole (grep)
regular expression (aka regexp) for the details
of the pattern specification.
agrep for approximate matching.
tolower, toupper and chartr
for character translations.
charmatch, pmatch, match.
apropos uses regexps and has nice 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'
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)