Strings
MATH-7360 Data Analysis
Dr. Xiang Ji @ Tulane University
Sep 30, 2020
String | r4ds chapter 14
stringr
stringr pacakge, by Hadley Wickham, provides utilities for handling strings.
Included in tidyverse.
library("tidyverse")## ── Attaching packages ───────────────────────────────────────────────────────────────────────────────────────────────────────── tidyverse 1.3.0 ──## ✓ ggplot2 3.3.2 ✓ purrr 0.3.4 ## ✓ tibble 3.0.3 ✓ dplyr 1.0.2 ## ✓ tidyr 1.1.2 ✓ stringr 1.4.0 ## ✓ readr 1.3.1 ✓ forcats 0.5.0## ── Conflicts ──────────────────────────────────────────────────────────────────────────────────────────────────────────── tidyverse_conflicts() ── ## x dplyr::filter() masks stats::filter() ## x dplyr::lag() masks stats::lag()# load htmlwidgets library(htmlwidgets)Main functions:
str_detect(string, pattern): Detect the presence or absence of a pattern in a string.
str_locate(string, pattern): Locate the first position of a pattern and return a matrix with start and end.
str_extract(string, pattern): Extracts text corresponding to the first match.
str_match(string, pattern): Extracts capture groups formed by () from the first match.
str_split(string, pattern): Splits string into pieces and returns a list of character vectors.
str_replace(string, pattern, replacement): Replaces the first matched pattern and returns a character vector.
Variants with an
_allsuffix will match more than 1 occurrence of the pattern in a given string.Most functions are vectorized.
Basics
Strings are enclosed by double quotes or single quotes:
string1 <- "This is a string" string2 <- 'If I want to include a "quote" inside a string, I use single quotes'Literal single or double quote:
double_quote <- "\"" # or '"' single_quote <- '\'' # or "'"
Printed representation:
x <- c("\"", "\\") x## [1] "\"" "\\"vs
cat:cat(x)## " \vs
writeLines():writeLines(x)## " ## \
Other special characters:
"\n"(new line),"\t"(tab), … Check?"'"for a complete list.
cat("a\"b")## a"bcat("a\tb")## a bcat("a\nb")## a ## bUnicode
x <- "\u00b5" x## [1] "µ"Character vector (vector of strings):
c("one", "two", "three")## [1] "one" "two" "three"
String length
Length of a single string:
str_length("R for data science")## [1] 18Lengths of a character vector:
str_length(c("a", "R for data science", NA))## [1] 1 18 NA
Combining strings
Combine two or more strings
str_c("x", "y")## [1] "xy"str_c("x", "y", "z")## [1] "xyz"Separator:
str_c("x", "y", sep = ", ")## [1] "x, y"
str_c()is vectorised:str_c("prefix-", c("a", "b", "c"), "-suffix")## [1] "prefix-a-suffix" "prefix-b-suffix" "prefix-c-suffix"Objects of length 0 are silently dropped:
name <- "Hadley" time_of_day <- "morning" birthday <- FALSE str_c( "Good ", time_of_day, " ", name, if (birthday) " and HAPPY BIRTHDAY", "." )## [1] "Good morning Hadley."
Combine a vector of strings:
str_c(c("x", "y", "z"))## [1] "x" "y" "z"str_c(c("x", "y", "z"), collapse = ", ")## [1] "x, y, z"
Subsetting strings
By position:
str_sub("Apple", 1, 3)## [1] "App"x <- c("Apple", "Banana", "Pear") str_sub(x, 1, 3)## [1] "App" "Ban" "Pea"Negative numbers count backwards from end:
str_sub(x, -3, -1)## [1] "ple" "ana" "ear"
Out of range:
str_sub("a", 1, 5)## [1] "a"str_sub("a", 2, 5)## [1] ""Assignment to a substring:
str_sub(x, 1, 1) <- str_to_lower(str_sub(x, 1, 1)) x## [1] "apple" "banana" "pear"
Matching patterns with regular expressions
str_view()shows the first match;
str_view_all()shows all matches.Match exact strings:
x <- c("apple", "banana", "pear") str_view(x, "an")str_view_all(x, "an")
.matches any character apart from a newline:str_view(x, ".a.")Regex escapes live on top of regular string escapes, so there needs to be two levels of escapes..
To match a literal.:# doesn't work because "a\.c" is treated as a regular expression str_view(c("abc", "a.c", "bef"), "a\.c")## Error: '\.' is an unrecognized escape in character string starting ""a\."
To match a literal# regular expression needs double escape str_view(c("abc", "a.c", "bef"), "a\\.c")\:str_view("a\\b", "\\\\")
Anchors
^matches the start of the string:x <- c("apple", "banana", "pear") str_view(x, "^a")$matches the end of the string:str_view(x, "a$")
To force a regular expression to only match a complete string:
x <- c("apple pie", "apple", "apple cake") str_view(x, "^apple$")
Alternation
[abc]: matches a, b, or c. Same as(a|b|c).[^abc]: matches anything except a, b, or c.
str_view(c("grey", "gray"), "gr(e|a)y")str_view(c("grey", "gray"), "gr[ea]y")
Repetition
?: 0 or 1
+: 1 or more
*: 0 or morex <- "1888 is the longest year in Roman numerals: MDCCCLXXXVIII" # match either C or CC, being greedy here str_view(x, "CC?")# greedy matches str_view(x, "CC+")# greedy matches str_view(x, 'C[LX]+')
Specify number of matches:
{n}: exactly n
{n,}: n or more
{,m}: at most m
{n,m}: between n and mstr_view(x, "C{2}")# greedy matches str_view(x, "C{2,}")# greedy matches str_view(x, "C{2,3}")
Greedy (default) vs lazy (put
?after repetition):# lazy matches str_view(x, 'C{2,3}?')# lazy matches str_view(x, 'C[LX]+?')What went wrong here?
text = "<div class='main'> <div> <a href='here.pdf'>Here!</a> </div> </div>" str_extract(text, "<div>.*</div>")## [1] "<div> <a href='here.pdf'>Here!</a> </div> </div>"If we add ? after a quantifier, the matching will be lazy (find the shortest possible match, not the longest).
str_extract(text, "<div>.*?</div>")## [1] "<div> <a href='here.pdf'>Here!</a> </div>"
Grouping and backreference
fruitis a character vector pre-defined instringrpackage:fruit## [1] "apple" "apricot" "avocado" ## [4] "banana" "bell pepper" "bilberry" ## [7] "blackberry" "blackcurrant" "blood orange" ## [10] "blueberry" "boysenberry" "breadfruit" ## [13] "canary melon" "cantaloupe" "cherimoya" ## [16] "cherry" "chili pepper" "clementine" ## [19] "cloudberry" "coconut" "cranberry" ## [22] "cucumber" "currant" "damson" ## [25] "date" "dragonfruit" "durian" ## [28] "eggplant" "elderberry" "feijoa" ## [31] "fig" "goji berry" "gooseberry" ## [34] "grape" "grapefruit" "guava" ## [37] "honeydew" "huckleberry" "jackfruit" ## [40] "jambul" "jujube" "kiwi fruit" ## [43] "kumquat" "lemon" "lime" ## [46] "loquat" "lychee" "mandarine" ## [49] "mango" "mulberry" "nectarine" ## [52] "nut" "olive" "orange" ## [55] "pamelo" "papaya" "passionfruit" ## [58] "peach" "pear" "persimmon" ## [61] "physalis" "pineapple" "plum" ## [64] "pomegranate" "pomelo" "purple mangosteen" ## [67] "quince" "raisin" "rambutan" ## [70] "raspberry" "redcurrant" "rock melon" ## [73] "salal berry" "satsuma" "star fruit" ## [76] "strawberry" "tamarillo" "tangerine" ## [79] "ugli fruit" "watermelon"Parentheses define groups, which can be back-referenced as
\1,\2, …# only show matched strings str_view(fruit, "(..)\\1", match = TRUE)
Detect matches
x <- c("apple", "banana", "pear") str_detect(x, "e")## [1] TRUE FALSE TRUEVector
wordscontains about 1000 commonly used words:length(words)## [1] 980head(words)## [1] "a" "able" "about" "absolute" "accept" "account"
# How many common words start with t? sum(str_detect(words, "^t"))## [1] 65# What proportion of common words end with a vowel? mean(str_detect(words, "[aeiou]$"))## [1] 0.2765306
Find words that end with
x:words[str_detect(words, "x$")]## [1] "box" "sex" "six" "tax"same as
str_subset(words, "x$")## [1] "box" "sex" "six" "tax"
Filter a data frame:
df <- tibble( word = words, i = seq_along(word) ) df %>% filter(str_detect(words, "x$"))## # A tibble: 4 x 2 ## word i ## <chr> <int> ## 1 box 108 ## 2 sex 747 ## 3 six 772 ## 4 tax 841
str_count()tells how many matches are found:x <- c("apple", "banana", "pear") str_count(x, "a")## [1] 1 3 1# On average, how many vowels per word? mean(str_count(words, "[aeiou]"))## [1] 1.991837Matches never overlap:
str_count("abababa", "aba")## [1] 2str_view_all("abababa", "aba")
Mutate a data frame:
df %>% mutate( vowels = str_count(word, "[aeiou]"), consonants = str_count(word, "[^aeiou]") )## # A tibble: 980 x 4 ## word i vowels consonants ## <chr> <int> <int> <int> ## 1 a 1 1 0 ## 2 able 2 2 2 ## 3 about 3 3 2 ## 4 absolute 4 4 4 ## 5 accept 5 2 4 ## 6 account 6 3 4 ## 7 achieve 7 4 3 ## 8 across 8 2 4 ## 9 act 9 1 2 ## 10 active 10 3 3 ## # … with 970 more rows
Extract matches
sentencesis a collection of 720 phrases:length(sentences)## [1] 720head(sentences)## [1] "The birch canoe slid on the smooth planks." ## [2] "Glue the sheet to the dark blue background." ## [3] "It's easy to tell the depth of a well." ## [4] "These days a chicken leg is a rare dish." ## [5] "Rice is often served in round bowls." ## [6] "The juice of lemons makes fine punch."Suppose we want to find all sentences that contain a colour.
Create a collection of colours:
(colours <- c("red", "orange", "yellow", "green", "blue", "purple"))## [1] "red" "orange" "yellow" "green" "blue" "purple"(colour_match <- str_c(colours, collapse = "|"))## [1] "red|orange|yellow|green|blue|purple"Select the sentences that contain a colour, and then extract the colour to figure out which one it is:
(has_colour <- str_subset(sentences, colour_match))## [1] "Glue the sheet to the dark blue background." ## [2] "Two blue fish swam in the tank." ## [3] "The colt reared and threw the tall rider." ## [4] "The wide road shimmered in the hot sun." ## [5] "See the cat glaring at the scared mouse." ## [6] "A wisp of cloud hung in the blue air." ## [7] "Leaves turn brown and yellow in the fall." ## [8] "He ordered peach pie with ice cream." ## [9] "Pure bred poodles have curls." ## [10] "The spot on the blotter was made by green ink." ## [11] "Mud was spattered on the front of his white shirt." ## [12] "The sofa cushion is red and of light weight." ## [13] "The sky that morning was clear and bright blue." ## [14] "Torn scraps littered the stone floor." ## [15] "The doctor cured him with these pills." ## [16] "The new girl was fired today at noon." ## [17] "The third act was dull and tired the players." ## [18] "A blue crane is a tall wading bird." ## [19] "Lire wires should be kept covered." ## [20] "It is hard to erase blue or red ink." ## [21] "The wreck occurred by the bank on Main Street." ## [22] "The lamp shone with a steady green flame." ## [23] "The box is held by a bright red snapper." ## [24] "The prince ordered his head chopped off." ## [25] "The houses are built of red clay bricks." ## [26] "The red tape bound the smuggled food." ## [27] "Nine men were hired to dig the ruins." ## [28] "The flint sputtered and lit a pine torch." ## [29] "Hedge apples may stain your hands green." ## [30] "The old pan was covered with hard fudge." ## [31] "The plant grew large and green in the window." ## [32] "The store walls were lined with colored frocks." ## [33] "The purple tie was ten years old." ## [34] "Bathe and relax in the cool green grass." ## [35] "The clan gathered on each dull night." ## [36] "The lake sparkled in the red hot sun." ## [37] "Mark the spot with a sign painted red." ## [38] "Smoke poured out of every crack." ## [39] "Serve the hot rum to the tired heroes." ## [40] "The couch cover and hall drapes were blue." ## [41] "He offered proof in the form of a lsrge chart." ## [42] "A man in a blue sweater sat at the desk." ## [43] "The sip of tea revives his tired friend." ## [44] "The door was barred, locked, and bolted as well." ## [45] "A thick coat of black paint covered all." ## [46] "The small red neon lamp went out." ## [47] "Paint the sockets in the wall dull green." ## [48] "Wake and rise, and step into the green outdoors." ## [49] "The green light in the brown box flickered." ## [50] "He put his last cartridge into the gun and fired." ## [51] "The ram scared the school children off." ## [52] "Tear a thin sheet from the yellow pad." ## [53] "Dimes showered down from all sides." ## [54] "The sky in the west is tinged with orange red." ## [55] "The red paper brightened the dim stage." ## [56] "The hail pattered on the burnt brown grass." ## [57] "The big red apple fell to the ground."(matches <- str_extract(has_colour, colour_match))## [1] "blue" "blue" "red" "red" "red" "blue" "yellow" "red" ## [9] "red" "green" "red" "red" "blue" "red" "red" "red" ## [17] "red" "blue" "red" "blue" "red" "green" "red" "red" ## [25] "red" "red" "red" "red" "green" "red" "green" "red" ## [33] "purple" "green" "red" "red" "red" "red" "red" "blue" ## [41] "red" "blue" "red" "red" "red" "red" "green" "green" ## [49] "green" "red" "red" "yellow" "red" "orange" "red" "red" ## [57] "red"
str_extract()only extracts the first match.more <- sentences[str_count(sentences, colour_match) > 1] str_view_all(more, colour_match)str_extract_all()extracts all matches:str_extract_all(more, colour_match)## [[1]] ## [1] "blue" "red" ## ## [[2]] ## [1] "green" "red" ## ## [[3]] ## [1] "orange" "red"
Setting
simplify = TRUEinstr_extract_all()will return a matrix with short matches expanded to the same length as the longest:str_extract_all(more, colour_match, simplify = TRUE)## [,1] [,2] ## [1,] "blue" "red" ## [2,] "green" "red" ## [3,] "orange" "red"x <- c("a", "a b", "a b c") str_extract_all(x, "[a-z]", simplify = TRUE)## [,1] [,2] [,3] ## [1,] "a" "" "" ## [2,] "a" "b" "" ## [3,] "a" "b" "c"
Grouped matches
str_extract()gives us the complete match:# why "([^ ]+)" match a word? noun <- "(a|the) ([^ ]+)" has_noun <- sentences %>% str_subset(noun) %>% head(10) has_noun %>% str_extract(noun)## [1] "the smooth" "the sheet" "the depth" "a chicken" "the parked" ## [6] "the sun" "the huge" "the ball" "the woman" "a helps"
str_match()gives each individual component:has_noun %>% str_match(noun)## [,1] [,2] [,3] ## [1,] "the smooth" "the" "smooth" ## [2,] "the sheet" "the" "sheet" ## [3,] "the depth" "the" "depth" ## [4,] "a chicken" "a" "chicken" ## [5,] "the parked" "the" "parked" ## [6,] "the sun" "the" "sun" ## [7,] "the huge" "the" "huge" ## [8,] "the ball" "the" "ball" ## [9,] "the woman" "the" "woman" ## [10,] "a helps" "a" "helps"
tidyr::extract()works with tibble:tibble(sentence = sentences) %>% tidyr::extract( sentence, c("article", "noun"), "(a|the) ([^ ]+)", remove = FALSE )## # A tibble: 720 x 3 ## sentence article noun ## <chr> <chr> <chr> ## 1 The birch canoe slid on the smooth planks. the smooth ## 2 Glue the sheet to the dark blue background. the sheet ## 3 It's easy to tell the depth of a well. the depth ## 4 These days a chicken leg is a rare dish. a chicken ## 5 Rice is often served in round bowls. <NA> <NA> ## 6 The juice of lemons makes fine punch. <NA> <NA> ## 7 The box was thrown beside the parked truck. the parked ## 8 The hogs were fed chopped corn and garbage. <NA> <NA> ## 9 Four hours of steady work faced us. <NA> <NA> ## 10 Large size in stockings is hard to sell. <NA> <NA> ## # … with 710 more rows
Replacing matches
Replace the first match:
x <- c("apple", "pear", "banana") str_replace(x, "[aeiou]", "-")## [1] "-pple" "p-ar" "b-nana"Replace all matches:
str_replace_all(x, "[aeiou]", "-")## [1] "-ppl-" "p--r" "b-n-n-"Multiple replacement:
x <- c("1 house", "2 cars", "3 people") str_replace_all(x, c("1" = "one", "2" = "two", "3" = "three"))## [1] "one house" "two cars" "three people"
Back-reference:
# flip the order of the second and third words sentences %>% str_replace("([^ ]+) ([^ ]+) ([^ ]+)", "\\1 \\3 \\2") %>% head(5)## [1] "The canoe birch slid on the smooth planks." ## [2] "Glue sheet the to the dark blue background." ## [3] "It's to easy tell the depth of a well." ## [4] "These a days chicken leg is a rare dish." ## [5] "Rice often is served in round bowls."
Splitting
Split a string up into pieces:
sentences %>% head(5) %>% str_split(" ")## [[1]] ## [1] "The" "birch" "canoe" "slid" "on" "the" "smooth" ## [8] "planks." ## ## [[2]] ## [1] "Glue" "the" "sheet" "to" "the" ## [6] "dark" "blue" "background." ## ## [[3]] ## [1] "It's" "easy" "to" "tell" "the" "depth" "of" "a" "well." ## ## [[4]] ## [1] "These" "days" "a" "chicken" "leg" "is" "a" ## [8] "rare" "dish." ## ## [[5]] ## [1] "Rice" "is" "often" "served" "in" "round" "bowls."
Use
simplify = TRUEto return a matrix:sentences %>% head(5) %>% str_split(" ", simplify = TRUE)## [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] ## [1,] "The" "birch" "canoe" "slid" "on" "the" "smooth" "planks." ## [2,] "Glue" "the" "sheet" "to" "the" "dark" "blue" "background." ## [3,] "It's" "easy" "to" "tell" "the" "depth" "of" "a" ## [4,] "These" "days" "a" "chicken" "leg" "is" "a" "rare" ## [5,] "Rice" "is" "often" "served" "in" "round" "bowls." "" ## [,9] ## [1,] "" ## [2,] "" ## [3,] "well." ## [4,] "dish." ## [5,] ""
More regular expressions
Character classes
A number of built-in convenience classes of characters:
.: Any character except new line\n.\s: White space.
\S: Not white space.
\d: Digit (0-9).\D: Not digit.\w: Word (A-Z, a-z, 0-9, or _).\W: Not word.
Example: How to match a telephone number with the form (###) ###-####?
text = c("apple", "(219) 733-8965", "(329) 293-8753") str_detect(text, "(\d\d\d) \d\d\d-\d\d\d\d")## Error: '\d' is an unrecognized escape in character string starting ""(\d"text = c("apple", "(219) 733-8965", "(329) 293-8753") str_detect(text, "(\\d\\d\\d) \\d\\d\\d-\\d\\d\\d\\d")## [1] FALSE FALSE FALSEstr_detect(text, "\\(\\d\\d\\d\\) \\d\\d\\d-\\d\\d\\d\\d")## [1] FALSE TRUE TRUE
Lists and ranges
- Lists and ranges:
[abc]: List (a or b or c)
[^abc]: Excluded list (not a or b or c)
[a-q]: Range lower case letter from a to q
[A-Q]: Range upper case letter from A to Q
[0-7]: Digit from 0 to 7
text = c("apple", "(219) 733-8965", "(329) 293-8753") str_replace_all(text, "[aeiou]", "") # strip all vowels## [1] "ppl" "(219) 733-8965" "(329) 293-8753"str_replace_all(text, "[13579]", "*")## [1] "apple" "(2**) ***-8*6*" "(*2*) 2**-8***"str_replace_all(text, "[1-5a-ep]", "*")## [1] "***l*" "(**9) 7**-896*" "(**9) *9*-87**"