Specify the Start or End of Strings with Anchors

Anchors define the position of a string where a match is supposed to be found: ^ denotes the start of the string, and $ denotes the end of the string. Consider the following examples.

Hello$ matches “Hello” that resides at the very end of a string.

library(stringr)x <- c("... Hello", "Hello world")

str_view_all(x, "Hello$") 

Output:
[1] │ ... <Hello>
[2] │ Hello world

str_extract_all(x, "Hello$", simplify = T) 

Output:
     [,1]
[1,] "Hello"
[2,] ""     

^[aA]pple matches “Apple” and “apple” that reside at the start of a string.

a <- c("apples are red", "Apples are juicy", "We love apples")

str_view_all(a, "^[aA]pple")

Output:
[1] │ <apple>s are red
[2] │ <Apple>s are juicy
[3] │ We love apples

str_extract_all(a, "^[aA]pple", simplify = T)

Output:
     [,1]
[1,] "apple"
[2,] "Apple"
[3,] ""     

It is important to note that when the caret ^ is used inside the square brackets [], it has a different meaning: it negates the character class, and no longer functions as the starting anchor, e.g., [^123] matches any character except “1”, “2”, or “3”.

Compare the following two examples.

x <- c("12a 53b",  "b**89  *_...")

Match consecutive digits 1 to 5 at the start of a string.

str_view_all(x, "^[1-5]+")

Output:
[1] │ <12>a 53b
[2] │ b**89  *_...

str_extract_all(x, "^[1-5]+", simplify = T)

Output:
     [,1]
[1,] "12"
[2,] ""  

Match any consecutive characters excluding 1 to 5.

str_view_all(x, "[^1-5]+")

Output:
[1] │ 12<a >53<b>
[2] │ <b**89  *_...>

str_extract_all(x, "[^1-5]+", simplify = T)

Output:
     [,1]           [,2]
[1,] "a "           "b"
[2,] "b**89  *_..." ""