R Lesson #7 - Searching characters
This lesson describes how to search for a pattern inside of a character vector. R has several functions that accept search patterns known as regular expressions. The different search functions are detailed below.Show output
123456-789101112131415-1617181920212223# FUNCTION OUTPUT_TYPE DESCRIPTION# strsplit list explode string at matches# gsub character replace matches# grep integer return index of matches# grepl logical TRUE if matched# gregexpr list position/length of match(es) x <- "The quick brown fox jumps over the lazy dog."y <- strsplit(x, " ")yy <- y[[1]]grep("u", y) # find "u"gsub("o", "OOOO", y) # replace "o" with "OOOO"grepl("u", y) # find "u"which(grepl("u", y)) # equivalent to grepstrsplit(y, "u") # returns a list # to figure out where the match occurred,# we can use gregexpr, which returns a# somewhat complicated list structureg <- gregexpr("u", y) # returns a liststr(g)g # position of matches or -1 (no match)attributes(g[[2]])$match.lengthregmatches(y, g) # get back the matches
Finding an exact pattern, also known as literal matching, is very fast. Depending on the input parameters, the grep function can return the indices of matches, the indices of non-matches, or the match value.
242526272829303132test1 <- c("abc", "Abc", "ABC", "ABCD")grep("abc", test1) # with the default argumentsgrep("abc", test1, value=TRUE)# with only letters the behavior is as expected,# special characters will be interpreted as# regular expressions (unless fixed=TRUE)grep("abc", test1, fixed=TRUE) # search exactly (fast)grep("abc", test1, ignore.case=TRUE)grep("abc", test1, invert=TRUE)
A regular expression is a grammer specifying the syntax for flexible pattern matching. Only the basics of regular expressions are described here.
33343536-37383940414243444546474849505152-53545556575859-6061626364656667-686970717273747576-7778798081828384858687?regex# these can get very advanced, for example:# "/^([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})$/"# can be used to match any email address # Repetition qualifiers:# "." matches anything once# "+" suffix means one or more times# "*" suffix means zero or more times# "?" suffix means zero or one timetest2 <- c("m+n", "man", "moon", "mn", "mooon", "mon")# zero or more 'o' characters between m and ngrep("mo*n", test2, value=TRUE)# one or more 'o' characters between m and ngrep("mo+n", test2, value=TRUE)# anything between m and n oncegrep("m.n", test2, value=TRUE)# zero or more characters between m and ngrep("m.*n", test2, value=TRUE)# zero or one 'o' characters between m and ngrep("mo?n", test2, value=TRUE) # special cases:# search for a special character (+) as isgrep("m+n", test2, value=TRUE)grep("m\\+n", test2, value=TRUE)grep("m+n", test2, fixed=TRUE, value=TRUE)# 2, 3 or 4 'o' characters between m and ngrep("mo{2,4}n", test2, value=TRUE) # Anchoring: beginning and ending# "^" means must start at the beginning# "$" means must finish at the endtest3 <- c("abc", "abc d", "abcd", "fabc")grep("abc", test3, value=TRUE)grep("^abc", test3, value=TRUE)grep("abc$", test3, value=TRUE)grep("^abc$", test3, value=TRUE) # Alternation constructs - either/ortest4 <- c("men", "man", "women", "moon", "maan", "mn")# the pipe "|" means ORgrep("e|a", test4, value=TRUE)# combining multiple special characters together# use parentheses to group part of the expressiongrep("m(e|a)+n", test4, value=TRUE)# parentheses can also group multiple charactersgrep("(en|an)", test4, value=TRUE) # Character classes - matching in a grouptest5 <- c("moon", "maan", "mn", "myn", "m_n", "mDn")# square brackets "[]" define a "character class":# only vowels located between m and ngrep("m[aeiou]*n", test5, value=TRUE)# the "^" in a character class means NOT# anything but a vowel located between m and ngrep("m[^aeiou]*n", test5, value=TRUE)# the "-" in a character class defines a range# only lowercase consonants located between m and ngrep("m[b-df-hj-np-tv-z]*n", test5, value=TRUE)
There are also built-in pattern classes that will match multiple digits, words, or other pre-defined patterns.
888990919293949596979899100101102103104105106107108109110-111112113114115116117118119120121# Built-in classes: \d, \wsentence <- "pi is approximately equal to 3.14 or 22/7."# match numbers without escaping "."nlist_matches1 <- gregexpr("\\d+.\\d+", sentence)nlist1 <- regmatches(sentence, nlist_matches1)nlist1# match numbers while escaping "."nlist_matches2 <- gregexpr("\\d+\\.\\d+", sentence)nlist2 <- regmatches(sentence, nlist_matches2)nlist2# list of all wordswlist_matches1 <- gregexpr("\\w+", sentence)wlist1 <- regmatches(sentence, wlist_matches1)wlist1# words with only letterswlist_matches2 <- gregexpr("[[:alpha:]]+", sentence)wlist2 <- regmatches(sentence, wlist_matches2)wlist2# words starting with an Upper case letterwlist_matches3 <- gregexpr("[[:upper:]][[:alpha:]]+", sentence)wlist3 <- regmatches(sentence, wlist_matches3)wlist3 # Word boundaries# words starting with an "a"wlist_matches4 <- gregexpr("\\<(a|A)[[:alpha:]]+", sentence)wlist4 <- regmatches(sentence, wlist_matches4)wlist4# words ending in a vowelwlist_matches5 <- gregexpr("[[:alpha:]]+[aeiou]\\>", sentence)wlist5 <- regmatches(sentence, wlist_matches5)wlist5
< Previous LessonNext Lesson >