Symbol Matches:

\t

Tab character.

\n

Newline character.

\\

Back slash character.

\w

Any "word" character: letters, digits, or underscore (_).

\W

Any "non-word" character

\s

Any white-space character: space, tab, newline, etc.

\S

Any non-white-space character.

\d

Any digit.

\D

Any non-digit.

\b

Any word boundary: a point in the string between a word and non-word character.

\B

Any non-word boundary: a point in the string between two characters that are both word characters, or are both non-word.

.

Any character.

^

The beginning of the string.

$

The end of the string.
Note: the backslash character may be used before special pattern characters such as ^, $, etc. in order to override their special meanings. For example, a dash (-) is a special character in patterns. To actually match a dash, use \-.
Pattern component Matches

P|Q

Either P or Q, where P and Q are patterns.

P+

One or more consecutive occurences of pattern P.

P*

Zero or more consecutive occurences of pattern P.

P?

Zero or one occurence of pattern P.

P{n}

Exactly n consecutive occurences of pattern P.

P{n,}

At least n consecutive occurences of pattern P.

P{n,m}

At least n, and at most m, consecutive occurences of pattern P.

[aeiou]

Any one of the characters a, e, i, o, or u (lowercase).

[a-z]

Any lower case letter.

[0-9]

Any digit.
Modifier Purpose
i Case-insensitive searching
g Global replace (works only with replace())