R regex - extract words beginning with @ symbol

Question

I'm trying to extract twitter handles from tweets using R's stringr package. For example, suppose I want to get all words in a vector that begin with "A". I can do this like so

library(stringr)

# Get all words that begin with "A"
str_extract_all(c("hAi", "hi Ahello Ame"), "(?<=\\b)A[^\\s]+")

[[1]]
character(0)

[[2]]
[1] "Ahello" "Ame"   

Great. Now let's try the same thing using "@" instead of "A"

str_extract_all(c("h@i", "hi @hello @me"), "(?<=\\b)\\@[^\\s]+")

[[1]]
[1] "@i"

[[2]]
character(0)

Why does this example give the opposite result that I was expecting and how can I fix it?

Answer 1

It looks like you probably mean

str_extract_all(c("h@i", "hi @hello @me", "@twitter"), "(?<=^|\\s)@[^\\s]+")
# [[1]]
# character(0)
# [[2]]
# [1] "@hello" "@me" 
# [[3]]
# [1] "@twitter"

The \b in a regular expression is a boundary and it occurs "Between two characters in the string, where one is a word character and the other is not a word character." see here. Since an space and "@" are both non-word characters, there is no boundary before the "@".

With this revision you match either the start of the string or values that come after spaces.