A.1 Metacharacters and POSIX character classes

• \w matches any word character (alphabet or number, or alphanumeric) and underscore, equivalent to [A-Za-z0-9_].
  
• \W is the opposite of \w that matches non-word character, or [^A-Za-z0-9_]
• \d matches any single digit number
  
• . matches any character except linebreaks, equivalent to [^\r\n] (Windows) or [\n] (Mac)
  
• \s matches any white space, including spaces, tabs and vertical tab, return and line breaks, equivalent to [:space:] in the following table.
  
• \S is the opposite of \s that matches any non-white character. [\s\S] is a common shorthand for matching everything, since . does not match linebreak.

And there are POSIX character classes.

class	description
[:alnum:]	alphabets or numbers, equivalent to [A-Za-z0-9]
[:alpha:]	alphabets, equivalent to [A-Za-z]
[:punct:]	punctuation
[:blank:]	space or tab, equivalent to [\t ]
[:space:]	any whitespace character including space [\f\n\r\t\v ]
[:print:]	any printable character, a similar expression is [:graph:] which excludes space
[:xdigit:]	any hexadecimal digit, equivalent to [F-Aa-f0-9]