\

The backslash character can have four different meanings depending on the context. It can:

• Stand for itself

• Quote the next character

• Introduce an operator

• Do nothing

*

Matches zero or more occurrences

+

Matches one or more occurrences

?

Matches zero or one occurrence

|

Alternation operator for specifying alternative matches

^

Matches the beginning of a string by default. In multiline mode, it matches the beginning of any line anywhere within the source string.

$

Matches the end of a string by default. In multiline mode, it matches the end of any line anywhere within the source string.

.

Matches any character in the supported character set except NULL

[ ]

Bracket expression for specifying a matching list that should match any one of the expressions represented in the list. A non-matching list expression begins with a circumflex (^) and specifies a list that matches any character except for the expressions represented in the list.

To specify a right bracket (]) in the bracket expression, place it first in the list (after the initial circumflex (^), if any).

To specify a hyphen in the bracket expression, place it first in the list (after the initial circumflex (^), if any), last in the list, or as an ending range point in a range expression.

( )

Grouping expression, treated as a single subexpression

{m}

Matches exactly m times

{m,}

Matches at least m times

{m,n}

Matches at least m times but no more than n times

\n

The backreference expression (n is a digit between 1 and 9) matches the n^th subexpression enclosed between '(' and ')' preceding the \n

[..]

Specifies one collation element, and can be a multicharacter element (for example, [.ch.] in Spanish)

[: :]

Specifies character classes (for example, [:alpha:]). It matches any character within the character class.

[==]

Specifies equivalence classes. For example, [=a=] matches all characters having base letter 'a'.