Data Science Desktop Survival Guide by Graham Williams
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Regexp Character Classes

20180608 A character class is a collection of characters that are in some way grouped together. We enclose the characters to be grouped within square backets []. The pattern then matches any one of the characters in the set. For example, the character class [0-9] matches any of the digits from 0 to 9.

	Character Class	Description
1	[0-9]	Digits
2	[a-z]	Lower-case letters
3	[A-Z]	Upper-case letters
4	[a-zA-Z]	Alphabetic characters
5	[^a-zA-Z]	Non-alphabetic characters
6	[a-zA-Z0-9]	Alphanumeric characters
7	[$\backslash$n$\backslash$t$\backslash$r$\backslash$f$\backslash$v]	Space characters
8	[!,:;`$\backslash$)}@-]$*+.?[^{$\vert$($\backslash$$\backslash$#%&˜_/$<$=$>$']	Punctuation characters

s <- c("abc12", "@#$", "345", "ABcd")
grep(pattern="[0-9]+", s, value=TRUE)

## [1] "abc12" "345"

grep(pattern="[A-Z]+", s, value=TRUE)

## [1] "ABcd"

grep(pattern="[^@#$]+", s, value=TRUE)

## [1] "abc12" "345"   "ABcd"

R also supports the use of POSIX character classes which are represented within [[]] (double braces).

grep(pattern="[[:alpha:]]", s, value=TRUE)

## [1] "abc12" "ABcd"

grep(pattern="[[:upper:]]", s, value=TRUE)

## [1] "ABcd"