grepl returns a logical vector (match or not for each element of x). grep(value = TRUE) returns a character vector containing the selected elements of x (after coercion, preserving names but no other attributes). Integer vector unless the input is a long vector, when it will be a double vector. Value grep(value = FALSE) returns a vector of the indices of the elements of x that yielded a match (or not, for invert = TRUE. Caseless matching with PERL = TRUE for non-ASCII characters depends on the PCRE library being compiled with 'Unicode property support': an external library might not be. Invalid inputs in the current locale are warned about up to 5 times. regexpr and gregexpr with perl = TRUE allow Python-style named captures, but not for long vector inputs. 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 useBytes = TRUE. It inhibits the conversion of inputs with marked encodings, and is forced if any input is found which is marked as "bytes" see Encoding). The main effect of useBytes is to avoid errors/warnings about invalid inputs and spurious matches in multibyte locales, but for regexpr it changes the interpretation of the output. For regexpr, gregexpr and regexec it is an error for pattern to be NA, otherwise NA is permitted and gives an NA match. If replacement contains backreferences which are not defined in pattern the result is undefined (but most often the backreference is taken to be ""). The two *sub functions differ only in that sub replaces only the first occurrence of a pattern whereas gsub replaces all occurrences. See the help pages on regular expression for details of the different types of regular expressions. fixed = FALSE, perl = FALSE: use POSIX 1003.2 extended regular expressions. perl = TRUE: use Perl-style regular expressions. Each of these functions (apart from regexec, which currently does not support Perl-style regular expressions) operates in one of three modes: 1. Details Arguments which should be character strings or character vectors are coerced to character if possible.
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