Expression Meaning Usage
\w Word characters: [a-zA-Z0-9_] \w+ or [\w#-]+
\d Digits: [0-9]
\s [\f\t\n\r ] form-feed, tab, newline, carriage return and SPACE
\W [^\w]
\D [^\d]
\S [^\s]
[:class:] POSIX character classes (alpha, alnum...) [:alpha:]+ or [[:alpha:]#-]+
\p{...} Unicode definitions (IsAlpha, IsLower, IsHebrew, ...) \p{IsHebrew}+ or [\p{IsHebrew}#!-]
\P{...} Negation of Unicode character class

See also perldoc perlre and perldoc perluniintro