Non-capturing group

To use parentheses without creating matching groups, you can create a non-capturing group with (?:).

x <- c("a gray cat", "a grey dog")
str_match(x, "gr(e|a)y")

##      [,1]   [,2]
## [1,] "gray" "a" 
## [2,] "grey" "e"

str_match(x, "gr(?:e|a)y")

##      [,1]  
## [1,] "gray"
## [2,] "grey"