matches character with unicode property, like <code>\p{Uppercase}</code>, see <a href="http://www.unicode.org/reports/tr44/#Property_Index.">unicode property list</a>.
Example <code>[[:digit:]AX]</code> matches all digits and A and X
letters, numbers, punctuation, and whitespace
space characters (basically equivalent to <code>\s</code>)
stringr multiline: match start of the input
stringr multiline: match end of the input
stringr multiline: match end of the input, but before final line terminator (if it exists)
Name of unicode character, e.g. <code>\N{grinning face}</code>
octal character where "ooo" is 1-3 octal digits
backreference to group #N, e.g. <code>\1</code>
postive lookahead; matches are followed by <code>abc</code> (non-capturing)
negative lookahead; matches are not followed by <code>abc</code> (non-capturing)
positive lookbehind; matches are preceeded by <code>abc</code> (non-capturing)
negative lookbehind; matches are not preceeded by <code>abc</code> (non-capturing)
atomic-match; no back-tracking if later matches fail