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Regular expression for specific combination of alphabets and numbers

I am trying to create regular expression for following type of strings:
combination of the prefix (XI/ YV/ XD/ YQ/ XZ), numerical digits only, and either no ‘Z’ or a ‘Z’ suffix.
For example, XD35Z should pass but XD01HW should not pass.
So far I tried following:
#"XD\d+Z?" - XD35Z passes but unfortunately it also works for XD01HW
#"XD\d+$Z" - XD01HW fails which is what I want but XD35Z also fails
I have also tried #"XD\d{1,}Z"? but it did not work
I need a single regex which will give me appropriate results for both types of strings.

Try this regex:
^(XI|YV|XD|YQ|XZ){1}\d+Z{0,1}$
I'm using quantifying braces to explicitly limit the allowed numbers of each character/group. And the ^ and $ anchors make sure that the regex matches only the whole line (string).
Broken into logical pieces this regex checks
^(XI|YV|XD|YQ|XZ){1} Starts with exactly one of the allowed prefixes
\d+ Is follow by one or more digits
Z{0,1}$ Ends with between 0 and 1 Z

You're misusing the $ which represents the end of the string in the Regex
It should be : #"^XD\d+Z?$" (notice that it appears at the end of the Regex, after the Z?)

The regex following the behaviour you want is:
^(XI|YV|XD|YQ|XZ)\d+Z?$
Explanation:
combination of the prefix (XI/ YV/ XD/ YQ/ XZ)
^(XI|YV|XD|YQ|XZ)
numerical digits only
\d+
‘Z’ or a ‘Z’ suffix
Z?$

Regular expression to match exactly the start of a string

I'm trying to build a regular expression in c# to check whether a string follow a specific format.
The format i want is: [digit][white space][dot][letters]
For example:
123 .abc follow the format
12345 .def follow the format
123 abc does not follow the format
I write this expression but it not works completelly well
Regex.IsMatch(exampleString, #"^\d+ .")

^ matches the start of the string, and you got it right.
\d+ matches one or more digits, and you got that one right as well.
A space in a regex matches a literal space, so that works too!
However, a . is a wildcard and will match any one character. You will need to escape it with a backslash like this if you want to match a literal period: \..
To match letters now, you can use [a-z]+ right after the period.
#"^\d+ \.[a-z]+"

The dot is a special character in regex, which matches any character (except, typically, newlines). To match a literal ., you need to escape it:
Regex.IsMatch(exampleString, #"^\d+ \.")
If you want to include the condition for the succeeding letters, use:
Regex.IsMatch(exampleString, #"^\d+ \.[A-Za-z]+$")

For you to get yours to match, keep in mind that the period in regular expressions is a special character that will match any character, so you'll need to escape that.
In addition, \s is a match for any white-space character (tabs, line breaks).
^\d+\s+ \..+
(untested)

Regex -Check if string start with zero + space or zero alone

I have this regular expression to math:
String start with a zero + white space + anything else
String is a zero
"0 fkvjdm" // Must Match
"0" // Must match
"0.56" // NOT match
Here is the regular expression I'm using:
^([0]$|([0]\s+.))
Is there a way to improve it? or, is it has a bug?
Thanks a lot for your help.
Environment
VS 2010 .net 4

First of all, there is no need to put 0 in a character class.
Secondly your regex will not match more than a single character after whitespace. As you don't have any quantifier on dot - . in 2nd part of your regex. To match more characters after whitespace, you should use .* (0 or more) or .+ (1 or more).
To improve in clarity, you can make use of optional quantifier here:
^0(\s+.*)?$

Seems like the second character is what causes the match to fail. If the second character is a period, then don't match; otherwise match. ?! says if what it matches succeeds, fail the whole match. Hence if the second character is a period, it will fail.
^0(?!\.).*

Regex for special case

I need to create a regex expression for the following scenario.
It can have only numbers and only one dot or comma.
First part can have one to three digits.
The second part can be a dot or a comma.
The third part can have one to two digits.
The valid scenarios are
123,12
123.12
123,1
123
12,12
12.12
1,12
1.12
1,1
1.1
1
I came up so far with this expression
\d{1,3}(?:[.,]\d{1,2})?
but it doesn't work well. For example the input is 11:11 is marked as valid.

You need to put anchors around your expression:
^\d{1,3}(?:[.,]\d{1,2})?$
^ will match the start of the string
$ will match the end of the string
If those anchors are missing, it will partially match on your string, since the last part is optional, means on "11:11" it can match on the digits before the colon and a second match will be on the digits after the colon.

Try to use ^ and $:
^\d{1,3}(?:[.,]\d{1,2})?$
^ The match must start at the beginning of the string or line.
$ The match must occur at the end of the string or before \n at the end of the line or string.

Minimum Length Regular Expression

I'm trying to write a regular expression that will validate that user input is greater than X number of non-whitespace characters. I'm basically trying to filter out begining and ending whitespace while still ensuring that the input is greater than X characters; the characters can be anything, just not whitespace (space, tab, return, newline).
This the regex I've been using, but it doesn't work:
\s.{10}.*\s
I'm using C# 4.0 (Asp.net Regular Expression Validator) btw if that matters.

It may be easier to not use regex at all:
input.Where(c => !char.IsWhiteSpace(c)).Count() > 10
If whitespace shouldn't count in the middle, then this will work:
(\s*(\S)\s*){10,}
If you don't care about whitespace in between non-whitespace characters, the other answers have that scenario covered.

This regular expression looks for eight or more characters between the first and last non-whitespace characters, ignoring leading and trailing whitespace:
\s*\S.{8,}\S\s*

If your trying to check (like in my case a phone number that contains 8 digits) , you need to refer to a number below the one you need.
(\s*(\S)\s*){7,}