R: Pattern Matching and Replacement

grep {base}

R Documentation

Pattern Matching and Replacement

Description

grep searches for matches to pattern (its first argument) within the vector x of character strings (second argument).

sub and gsub perform replacement of matches determined by regular expression matching. The two functions differ only in that sub replaces only the first occurence of a pattern whereas gsub replaces all occurences.

The regular expressions used are those specified by POSIX 1003.2, either extended or basic, depending on the value of the extended argument.

Usage

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)

Arguments

pattern

character string containing a regular expression to be matched in the vector of character string vec.

x

a vector of character strings where matches are sought.

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.

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.

replacement

a replacement for matched pattern in sub and gsub.

Note

This function is based on the regex regular expression library written by Henry Spencer of the University of Toronto.

Examples

grep("[a-z]", letters)

## 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
all( 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