Why is it that in R when using a single square brackets here returns FALSE, whereas using double square brackets returns TRUE? Example:
> grepl('[:alpha:]', 'AA')
[1] FALSE
> grepl('[[:alpha:]]', 'AA')
[1] TRUE
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pd441
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1`[:alpha:]` A posix character class may only appear inside a character class. – Pushpesh Kumar Rajwanshi Mar 27 '19 at 14:35
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`[:alpha:]` only works in ICU regex, with `stringr` functions. Basically, in almost all flavors supporting POSIX character classes, the POSIX character classes must be used inside bracket expressions. – Wiktor Stribiżew Mar 27 '19 at 14:36
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[[:alpha:]] is a specific character class unique to regex in R. Check out character classes here
https://www.rstudio.com/wp-content/uploads/2016/09/RegExCheatsheet.pdf
M. Stolte
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1The mere fact they are called *POSIX* character classes implies they are not unique to R regex. And you may use `[:alpha:]`, too, but with `stringr` functions that are powered with the ICU regex library. – Wiktor Stribiżew Mar 27 '19 at 14:39