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Regular Expression oddity, why does this happen?

This simple regular expression matches the text of Movie. Am I wrong in reading this as "Q repeated zero or more times"? Why does it match, shouldn't it return false?
public class Program
{
private static void Main(string[] args)
{
Regex regex = new Regex("Q*");
string input = "Movie";
if (regex.IsMatch(input))
{
Console.WriteLine("Yup.");
}
else
{
Console.WriteLine("Nope.");
}
}
}

As you are saying correctly, it means “Q repeated zero or more times”. I this case, it’s zero times, so you are essentially trying to match "" in your input string. As IsMatch doesn’t care where it matches, it can match the empty string anywhere within your input string, so it returns true.
If you want to make sure that the whole input string has to match, you can add ^ and $: "^Q*$".
Regex regex = new Regex("^Q*$");
Console.WriteLine(regex.IsMatch("Movie")); // false
Console.WriteLine(regex.IsMatch("QQQ")); // true
Console.WriteLine(regex.IsMatch("")); // true

You are right in reading this regex as Q repeated 0 or more times. The thing with that is the 0. When you try a regex, it will try to find any successful match.
The only way for the regex to match the string is to try matching an empty string (0 times), which appears anywhere in-between the matches, and if you didn't know that before, yes, regex can match empty strings between characters. You can try:
(Q*)
To get a capture group and use .Matches and Groups[1].Value to see what has been captured. You'll see that it's an empty string.
Usually, if you want to check the existence of a character, you don't use regex, but use .Contains. Otherwise, if you do want to use regex, you'd drop the quantifier, or use one which matches at least one particular character.

Regex problems with equal sign?

In C# I'm trying to validate a string that looks like:
I#paramname='test'
or
O#paramname=2827
Here is my code:
string t1 = "I#parameter='test'";
string r = #"^([Ii]|[Oo])#\w=\w";
var re = new Regex(r);
If I take the "=\w" off the end or variable r I get True. If I add an "=\w" after the \w it's False. I want the characters between # and = to be able to be any alphanumeric value. Anything after the = sign can have alphanumeric and ' (single quotes). What am I doing wrong here. I very rarely have used regular expressions and normally can find example, this is custom format though and even with cheatsheets I'm having issues.

^([Ii]|[Oo])#\w+=(?<q>'?)[\w\d]+\k<q>$
Regular expression:
^ start of line
([Ii]|[Oo]) either (I or i) or (O or o)
\w+ 1 or more word characters
= equals sign
(?<q>'?) capture 0 or 1 quotes in named group q
[\w\d]+ 1 or more word or digit characters
\k<q> repeat of what was captured in named group q
$ end of line

use \w+ instead of \w to one character or more. Or \w* to get zero or more:
Try this: Live demo
^([Ii]|[Oo])#\w+=\'*\w+\'*

If you are being a bit more strict with using paramname:
^([Ii]|[Oo])#paramname=[']?[\w]+[']?
Here is a demo

You could try something like this:
Regex rx = new Regex( #"^([IO])#(\w+)=(.*)$" , RegexOptions.IgnoreCase ) ;
Match group 1 will give you the value of I or O (the parameter direction?)
Match group 2 will give you the name of the parameter
Match group 3 will give you the value of the parameter
You could be stricter about the 3rd group and match it as
(([^']+)|('(('')|([^']+))*'))
The first alternative matches 1 or more non quoted character; the second alternative match a quoted string literal with any internal (embedded) quotes escape by doubling them, so it would match things like
'' (the empty string
'foo bar'
'That''s All, Folks!'

regex ismatch logic with special character

I know this statement should return false as expected
Regex.IsMatch("+", #"[a-zA-Z0-9]")
but why these statements matches although they shouldn't (from my understanding)
Regex.IsMatch("C++", #"[a-zA-Z0-9]")
Regex.IsMatch("C++", #"[a-zA-Z0-9]+")

Those are matches because you don't match the entire string. They will match the C in C++.
Use ^ and $ to match the beginning and end of the string:
bool onlyAlphaNumeric = Regex.IsMatch("C++", #"^[a-zA-Z0-9]+$"); // will be false

Trouble creating a Regex expression

I'm trying to create a regex expression what will accept a certain format of command. The pattern is as follows:
Can start with a $ and have two following value 0-9,A-F,a-f (ie: $00 - $FF)
or
Can be any value except for "&<>'/"
*if the value start with $ the next two values after need to be a valid hex value from 00-ff
So far I have this
Regex correctValue = new Regex("($[0-9a-fA-F][0-9a-fA-F])");
Any help will be greatly appreciated!

You just need to add "\" symbol before your "$" and it works:
string input = "$00";
Match m = Regex.Match(input, #"^\$[0-9a-fA-F][0-9a-fA-F]$");
if (m.Success)
{
foreach (Group g in m.Groups)
Console.WriteLine(g.Value);
}
else
Console.WriteLine("Didn't match");

If I'm following you correctly, the net result you're looking for is any value that is not in the list "&<>'/", since any combination of $ and two alphanumeric characters would also not be in that list. Thus you could make your expression:
Regex correctValue = new Regex("[^&<>'/]");
Update: But just in case you do need to know how to properly match the $00 - $FF, this would do the trick:
Regex correctValue = new Regex("\$[0-9A-Fa-f]{2}");

In Regular Expression $ use for Anchor assertion, and means:
The match must occur at the end of the string or before \n at the end of the line or string.
try using [$] (Character Class for single character) or \$ (Character Escape) instead.

Using Regex to determine if string contains a repeated sequence of a particular substring with comma separators and nothing else

I want to find if a string contains a repeated sequence of a known substring (with comma separators) and nothing else and return true if this is the case; otherwise false. For example: the substring is "0,8"
String A: "0,8,0,8,0,8,0,8" returns true
String B: "0,8,0,8,1,0,8,0" returns false because of '1'
I tried using the C# string functions Contains but it does not suit my requirements. I am totally new to regular expression but I feel it should be powerful enough to do this. What RegEx should I use to do this?

The pattern for a string containing nothing but a repeated number of a given substring (possibly zero of them, resulting in an empty string) is \A(?:substring goes here)*\z. The \A matches the beginning of the string, the \z the end of the string, and the (?:...)* matches 0 or more copies of anything matching the thing between the colon and the close parenthesis.
But your string doesn't actually match \A(?:0,8)*\z, because of the extra commas; an example that would match is "0,80,80,80,8". You need to account for the commas explicitly with something like \A0,8(?:,0,8)*\z.
You can build such a thing in C# thus:
string OkSubstring = "0,8";
string aOk = "0,8,0,8,0,8,0,8";
string bOK = "0,8,0,8,1,0,8,0";
Regex OkRegex = new Regex( #"\A" + OkSubstring + "(?:," + OkSubstring + #")*\z" );
OkRegex.isMatch(aOK); // True
OkRegex.isMatch(bOK); // False
That hard-codes the comma-delimiter; you could make it more general. Or maybe you just need the literal regex. Either way, that's the pattern you need.
EDIT Changed the anchors per Mike Samuel's suggestion.