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Here is a short and general regular expressions reference found here.

Character Definition Example

Most characters

Characters other than  . $ ^ { [ ( | ) * + ? \  match themselves. If you need to match these special characters - precede them by \ abc matches abc
\? matches ?

^

The pattern has to appear at the beginning of a string. ^cat matches any string that begins with cat

$

The pattern has to appear at the end of a string. cat$ matches any string that ends with cat

.

Matches any character. cat. matches catT and cat2 but not catty

[]

Bracket expression. Matches one of any characters enclosed. gr[ae]y matches gray or grey

[^]

Negates a bracket expression. Matches one of any characters EXCEPT those enclosed. 1[^02] matches 13 but not 10 or 12

[-]

Range. Matches any characters within the range. [1-9] matches any single digit EXCEPT 0

?

Preceeding item must match one or zero times. colou?r matches color or colour but not colouur

+

Preceeding item must match one or more times. be+ matches be or bee but not b

*

Preceeding item must match zero or more times. be* matches b or be or beeeeeeeeee

()

Parentheses. Creates a substring or item that metacharacters can be applied to a(bee)?t matches at or abeet but not abet

{n}

Bound. Specifies exact number of times for the preceeding item to match. [0-9]{3} matches any three digits

{n,}

Bound. Specifies minimum number of times for the preceeding item to match. [0-9]{3,} matches any three or more digits

{n,m}

Bound. Specifies minimum and maximum number of times for the preceeding item to match. [0-9]{3,5} matches any three, four, or five digits

|

Alternation. One of the alternatives has to match. July (first|1st|1) will match July 1st but not July 2

\n

The newline character. (ASCII 10) \n matches a newline

\s

A single whitespace character. a\sb matches a b but not ab

\d

A single digit character. a\db matches a2b but not acb

For more complete and .net-specific reference see, for example, Regex hero's reference.
For more complex but also even more complete reference - see the primary source - Regular Expression Language Elements.
For information about regular expressions in general, their theoretical meaning - read Wikipedia!