stringr is a string handling package written by Hadley Wickham that is designed to improve / simplify string handling in R. Most are wrappers for base R 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. |
Many of these functions have variants with an _all suffix which will match more than one occurrence of the pattern in a given string.
text = c("The","quick","brown","fox","jumps","over","the","lazy","dog")str_detect(text,"quick")## [1] FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSEstr_detect(text,"o")## [1] FALSE FALSE TRUE TRUE FALSE TRUE FALSE FALSE TRUEstr_detect(text,"row")## [1] FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSEAn escape character is a character which results in an alternative interpretation of the following character(s). These vary from language to language but most often \ is the escape character and it is followed by a single character.
Some common examples:
\' |
single quote |
\" |
double quote |
\\ |
backslash |
\n |
new line |
\r |
carriage return |
\t |
tab |
\b |
backspace |
\f |
form feed |
The power of regular expressions comes from the ability to use a number of special metacharacters that modify how pattern matching is performed.
. ^ $ * + ? { } [ ] \ | ( )Because of their special properties they cannot be matched directly, if you need to match one of them you need to escape it first (precede it by \). One problem is that regex escapes live on top of regular string escapes, so the following causes an error
str_detect("abc[def","\[")We actually need to prefix [ with \\ because \ is also an escape for normal character strings.
str_detect("abc[def","\\[")## [1] TRUEHow do you detect if a string contains a \ character?
cat("abc\\def\n")## abc\defstr_detect("abc\\def","\\\\")## [1] TRUESometimes we want to specify that our pattern ours at a particular location, we can do this using anchors.
^ or \A |
Start of string |
$ or \Z |
End of string |
\b |
Word boundary |
\B |
Not word boundary |
text = "the quick brown fox jumps over the lazy dog"str_replace(text,"^the","---")## [1] "--- quick brown fox jumps over the lazy dog"str_replace(text,"^dog","---")## [1] "the quick brown fox jumps over the lazy dog"str_replace(text,"the$","---")## [1] "the quick brown fox jumps over the lazy dog"str_replace(text,"dog$","---")## [1] "the quick brown fox jumps over the lazy ---"str_replace(text,"\\Athe","---")## [1] "--- quick brown fox jumps over the lazy dog"str_replace(text,"dog\\Z","---")## [1] "the quick brown fox jumps over the lazy ---"str_replace(text,"\\brow\\b","---")## [1] "the quick brown fox jumps over the lazy dog"str_replace(text,"\\Brow\\B","---")## [1] "the quick b---n fox jumps over the lazy dog"When we want to match whole classes of characters at a time there are a number of convenience patterns built in,
\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 |
We can also specify our own character groups through the construction of ranges
. |
Any character except new line () |
[abc] |
Range (a or b or c) |
[^abc] |
Not (a or b or c) |
[a-q] |
Lower case letter from a to q |
[A-Q] |
Upper case letter from A to Q |
[0-7] |
Digit from 0 to 7 |
Attached to literals, character classes, ranges or groups to match repeats.
* |
Match 0 or more |
+ |
Match 1 or more |
? |
Match 0 or 1 |
{3} |
Match Exactly 3 |
{3,} |
Match 3 or more |
{3,5} |
Match 3, 4 or 5 |
Add a ? to a quantifier to make it ungreedy.
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>"str_extract(text, "<div>.*?</div>")## [1] "<div> <a href='here.pdf'>Here!</a> </div>"Group together parts of a regular expression for modification or capture.
(a|b) |
match literal a or b, group either |
a(bc)? |
match literal a or abc, group bc or “” |
(?:abc) |
Non-capturing group |
| `(abc)def(hig) | match abcdefhig, group abc and hig |
text = c("apple", "219 733 8965", "329-293-8753", "Work: 579-499-7527; Home: 543.355.3679")
phone = "([2-9][0-9]{2})[- .]([0-9]{3})[- .]([0-9]{4})"str_extract(text, phone)## [1] NA "219 733 8965" "329-293-8753" "579-499-7527"str_extract_all(text, phone)## [[1]]
## character(0)
##
## [[2]]
## [1] "219 733 8965"
##
## [[3]]
## [1] "329-293-8753"
##
## [[4]]
## [1] "579-499-7527" "543.355.3679"str_match(text[2:4], phone)## [,1] [,2] [,3] [,4]
## [1,] "219 733 8965" "219" "733" "8965"
## [2,] "329-293-8753" "329" "293" "8753"
## [3,] "579-499-7527" "579" "499" "7527"str_match_all(text[2:4], phone)## [[1]]
## [,1] [,2] [,3] [,4]
## [1,] "219 733 8965" "219" "733" "8965"
##
## [[2]]
## [,1] [,2] [,3] [,4]
## [1,] "329-293-8753" "329" "293" "8753"
##
## [[3]]
## [,1] [,2] [,3] [,4]
## [1,] "579-499-7527" "579" "499" "7527"
## [2,] "543.355.3679" "543" "355" "3679"Validating an email address:
(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])For the following vector of randomly generated names, write a regular expression that,
detects if the person’s first name starts with a vowel (a,e,i,o,u)
detects if the person’s last name starts with a vowel
detects if either the person’s first or last name start with a vowel
detects if neither the person’s first nor last name start with a vowel
## c("Jarvez Nguyen", "Kaleb Cho", "Kyana Bradley", "Malik La",
## "Mario Morales", "Trong Nguyen", "Abigail Ohmie", "Anthony Kessenich",
## "Laura Gonzales", "Thomas Vue", "Nicolasa Soltero", "Sanjana Stuckey",
## "Destiny Langley", "Brianna Ortiz", "Condeladio Owens", "Joshua Wilson",
## "Abigail Adu", "Cassidy Chavez", "Megan Dorsey", "Maomao Brown"
## )Write one or more regular expressions to extract the data contained in the xml file below:
<breakfast_menu>
<food>
<name>Belgian Waffles</name>
<price>$5.95</price>
<calories>650</calories>
<addon>
<name>Strawberries</name>
<price>$2.00</price>
<calories>250</calories>
</addon>
</food>
<food>
<name>French Toast</name>
<price>$4.50</price>
<calories>600</calories>
<addon>
<name>Strawberries</name>
<price>$2.00</price>
<calories>250</calories>
</addon>
</food>
</breakfast_menu>Hadley Wickham - stringr vigneete
David Child - RegEx Cheat Sheet