Regular expressions and stringr

stringr

stringr

stringr is a string handling package written by Hadley Wickham that is designed to improve / simplify string handling in R.

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.

Regular Expressions

Simple Matching

text = c("The","quick","brown","fox","jumps","over","the","lazy","dog")
str_detect(text,"quick")
## [1] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
str_detect(text,"o")
## [1] FALSE FALSE  TRUE  TRUE FALSE  TRUE FALSE FALSE  TRUE
str_detect(text,"row")
## [1] FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE

Metacharacters

The power of regular expressions comes from the ability to use a number of special metacharacters that modify how the 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

str_detect("abc[def","[")
## Error in stri_detect_regex(string, pattern, opts_regex = attr(pattern, : Missing closing bracket on a bracket expression. (U_REGEX_MISSING_CLOSE_BRACKET)
str_detect("abc[def","\\[")
## [1] TRUE

Note we actually need to prefix [ with \\ because \ is also a escape character for normal strings.

Anchors

^ or \A Start of string
$ or \Z End of string
\b Word boundary
\B Not word boundary

Anchor Examples

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,"\\bbrown\\b","-")
## [1] "The quick - fox jumps over the lazy dog"
str_replace(text,"\\Brow\\B","-")
## [1] "The quick b-n fox jumps over the lazy dog"

Character Classes

\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 |

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

Quantifiers

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

Greedy vs ungreedy matching

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>"

Groups

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

Group Examples

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"

Matching

str_match(text, phone)
##      [,1]           [,2]  [,3]  [,4]  
## [1,] NA             NA    NA    NA    
## [2,] "219 733 8965" "219" "733" "8965"
## [3,] "329-293-8753" "329" "293" "8753"
## [4,] "579-499-7527" "579" "499" "7527"
str_match_all(text, phone)
## [[1]]
##      [,1] [,2] [,3] [,4]
## 
## [[2]]
##      [,1]           [,2]  [,3]  [,4]  
## [1,] "219 733 8965" "219" "733" "8965"
## 
## [[3]]
##      [,1]           [,2]  [,3]  [,4]  
## [1,] "329-293-8753" "329" "293" "8753"
## 
## [[4]]
##      [,1]           [,2]  [,3]  [,4]  
## [1,] "579-499-7527" "579" "499" "7527"
## [2,] "543.355.3679" "543" "355" "3679"

Exercise

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>

Acknowledgments

Acknowledgments