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Regex pattern with dot and quotation mark

How can I declare regex pattern in c#. Pattern starts with quotation mark and later is url address, some thing like below
\"www\.mypage\.pl //<- this is my pattern
string pattern = ? //todo: what should I put there

using verbatim strings:
string pattern = #"""www\.mypage\.pl";

You can use \ backslash to escape the regex semantic and revert to the actual literal. If you want to escape
"www.mypage.pl
you can use
\"[0-9a-zA-Z]*\.[0-9a-zA-Z]*\.[0-9a-zA-Z]*
^ this is not to escape the regex but the eventual string
if you use single quotes you don't need to escape the quotes!
Note that [0-9a-zA-Z]* will require some more characters like % and - and _ to capture all cases according to an RFC about URL syntax I can't produce now (it's easy to find online).
If you want to escape
\"www\.mypage\.pl
you have to escape the escapes as well:
\\\"[0-9a-zA-Z]*\\\.[0-9a-zA-Z]*\\\.[0-9a-zA-Z]*

Other answers have already handled (very nicely) how to put the phrase itself into a string.
To use it, you need to refer to the System.Text.RegularExpressions namespace, documentation for which is over at MSDN. As a (very) quick example:
System.Text.RegularExpressions.Regex.Replace(content,
regexPattern, newMatchContent);
will replace matches for the regular expression regexPattern in content with newMatchContent, and return the result.

Regex.Replace not working

I am having a strange issue with Regex.Replace
string test = "if the other party is in material breach of such contract and, after <span style=\"background-color:#ffff00;\">forty-five (45)</span> calendar days notice of such breach";
string final = Regex.Replace(test, "forty-five (45)", "forty-five (46)", RegexOptions.IgnoreCase);
the "final" string still shows "forty-five (45)". Any idea why? I am assuming it has to do something with the tag. How do I fix this?
Thanks

Escape the parenthesis. Depending on the language, might require two back slashes.
string final = Regex.Replace(test, "forty-five \(45\)", "forty-five (46)", RegexOptions.IgnoreCase);
Basically, parenthesis are defined to mean something, and by escaping the characters, you are telling regex to use the parenthesis character, and not the meaning.
Better yet, why are you using a Regex to do this at all? Try just doing a normal string replacement.
string final = test.Replace("forty-five (45)", "forty-six (46)")

Parentheses are special in regular expressions. They delimit a group, to allow for things such as alternation. For example, the regular expression foo(bar|bat)baz matches:
foo, followed by
either bar OR bat, followed by
baz
So, a regular expression like foo(bar) will never match the literal string foo(bar). What it will match is the literal string foobar. Consequently, you need to escape the metacharacters. In C#, this should do you:
string final = Regex.Replace(test, #"forty-five \(45\)", "forty-five (46)", RegexOptions.IgnoreCase);
The #-quoted string helps avoid headaches from excessive backslashes. Without it, you'd have to write "forty-five \(45\)".

If you are unable to escape the parenthesis, put them in a character class:
forty-five [(]45[)]

C# Using Regular Expressions

The following works in vb.net, and basically only allows characters on a standard US Keyboard. Any other character pasted gets deleted. I use the following regular expression code:
"[^A-Za-z0-9\[\{\}\]`~!##$%\^&*\(\)_\-+=\\/:;'""<>,\.|? ]", "")
However when I try to use it in C# it won't work, I used '\' as a escape sequence. C# seems a bit different when it comes to escape sequences? Any help would be appreciated.

Prefix the string with #. That's it. From there you can use the regex string from VB as is (including doubling up on the " character).
// Note: exact same string you're using, only with a # verbatim prefix.
string regex = #"[^A-Za-z0-9\[\{\}\]`~!##$%\^&*\(\)_\-+=\\/:;'""<>,\.|? ]";
string crazy = "hĀečlĤlŁoźtƢhǣeǮrȡe";
Console.WriteLine(Regex.Replace(crazy, regex, ""));
Output:
hellothere

Prefix your string with "#" and prefix quotes within the string with "\".
I.e. this string
abc\def"hij
in C# would be encoded as
#"abc\def\"hij"

You need to escape your " character. Do this by putting a \ before your " character.
"[^A-Za-z0-9[{}]`~!##$%\^&*()_-+=\/:;'""<>,.|? ]"
should become
"[^A-Za-z0-9[{}]`~!##$%\^&*()_-+=\/:;'\"\"<>,.|? ]"
If you use the #prefix before this, it will treat the backslash literally instead of an escape character and you wont get the desired result.

Escape your characters:
"[^A-Za-z0-9[{}]`~!##$%\^&*()_-+=\\/:;'\"<>,.|? ]"
A good tool for regular expression design and testing (free) is:
http://www.radsoftware.com.au/regexdesigner/

You need to escape you regex for use in C#
[^A-Za-z0-9\[\{\}\]`~!##$%\^&*\(\)_\-+=\\/:;'\"<>,\.|? ]
Try this one!

How do I replace an actual asterisk character (*) in a Regex expression?

I have a statement:
I have a string such as
content = "* test *"
I want to search and replace it with so when I am done the string contains this:
content = "(*) test (*)"
My code is:
content = Regex.Replace(content, "*", "(*)");
But this causes an error in C# because it thinks that the * is part of the Regular Expressions Syntax.
How can I modify this code so it changes all asterisks in my string to (*) instead without causing a runtime error?

Since * is a regex metacharacter, when you need it as a literal asterisk outside of a character class definition, it needs to be escaped with \ to \*.
In C#, you can write this as "\\*" or #"\*".
C# should also have a general purpose "quoting" method so that you can quote an arbitrary string and match it as a literal.
See also
Regular expressions and escaping special characters
Full list of what needs to be escaped, where/when

You can escape it:
\*

You don't need a regular expression in this simple scenario. You can use String.Replace:
content = content.Replace("*", "(*)");

Use Regex.Escape() It will take all of the string and make it into something you can use as part of a regex.

Use \\* instead of * in regex.replace call

Replace '$' using Regex

I want to replace
! = change
# = static(does not)
$ = Want to replace
I got a string like this #!$!
How do I replace the $ with something else?
EDIT: I need to use Regex as the string may appear anywhere!

You don't need a regular expression, just use the String.Replace method:
String result = input.Replace("$", "somethingElse");
As a side note: The way that you would do this with a regular expression would be like this:
String result = Regex.Replace(input, #"\$", "somethingElse");
Notice that I have escaped the $ with a backslash since $ usually means match the end of the string.

Take a look at System.Text.RegularExpressions.Regex.Replace method.
Regex.Replace("#!$!", "!(.*)!", "replacement value");

Why do you want some RegExp for string replacement. You can just use string.Replace() fundtion.

also, check out Rubular, a great RegEx Tester.

Using the String class' .Replace() method would do the trick but, if you really want to use RegEx, this is a great RegEx site that I use quite often.
Regular Expression Library
You should be able to find what you're looking for there.