Symbol Description
\d digit. Same as [0-9]
\D not a digit
\w word character - same as [a-z]
\W not word character
[a-z] Lowercase alphabet
[a-zA-Z] Any letter of the alphabet, any case.
[a-zA-Z0-9] Alphanumeric
\s Whitespace
\S Not whitespace
\b Word boundary, like space or punctuation
. Any character, except a newline
:alpha: Letter
:digit: Digit
:space: Space
:blank: Space or tab, but not a newline.

You can also use word boundary to pick up letters. e.g. \b\s+\b can be used to find spaces surrounded by letters, i.e. ignoring part of a line starting with spaces.

Escaped literal characters:

Symbol Description
\. A literal dot
\n A newline character
\t A tab character
\\ A literal backspace
\$ A literal dollar sign
\? A literal question mark
\( A literal bracket
\[ A literal bracket