Develop Reference

C# regular Expression that allows only alphabets and numbers but not any special characters or spaces

I need to validate a textbox that should accept strings like 'ab123cd' , 'xy12345', 'a567891'. How can I write a regular expression to meet this requirement? The Length of the accepting string should not exceed 7 characters. Should not allow any Special characters or spaces within the string.
while(textbox.text does not match the requirement)
{
MessageBox.Show("Please enter Valid string");
prompt.ShowDialog(); //displaying a Dialog box that shows a textbox
}
Also help me writing the code as shown above.
Thank You !!

Edit:
UPDATED
You can try this
^(?=.*[a-zA-Z])(?=.*[0-9])[a-zA-Z0-9]{1,7}$
Explanation:
^ marks start position
(?=.*[a-zA-Z]) looks ahead to see if there is any alphabet
(?=.*[0-9]) looks ahead to see if there is any number
[a-zA-Z0-9] means any character between a-z , A-Z or 0-9
{1,7} means can only occur 1-7 times
$ marks end of the string
Demo

You can use
^\w{1,7}$
Means between one and seven alphanumerical chars. You can easily adjust length limits.
If you prefer a greater control other allowed characters, use :
^[a-zA-Z0-9]{1,7}$, because \w is more permissive so be careful
For C#, you can use the Regex.IsMatch method : Regex.IsMatch méthode (System.Text.RegularExpressions) : https://msdn.microsoft.com/fr-fr/library/system.text.regularexpressions.regex.ismatch(v=vs.110).aspx
Very simple to use.

Restrict all the special characters through a regex

There is a long set of characters that are not allowed to validate an input box of winform app.
So i figured that rather than making the long list that are not allowed make the shorter one that are allowed.
The set that is allowed are (a-z,A-Z, 0-9,#,.) .Rest every thing that can be entered are not allowed.
This is the regex that i have made for this.
Regex.IsMatch(textBox1.Text, #"[#\.\w]+$")
It seem to work in some cases but when i enter the data in this format normal character or number special character normal character or number it seems to break few example ee(vv, 55)44,aba&3B.
Not able to figure out whats wrong in this.

Your regex is not valid, because you don't validate all string, but the last part.
You should start it with ^ - beginning of the line symbol.
Regex.IsMatch(textBox1.Text, #"^[\w#.]*$")
\w also means letters in every language, so it will validate the string "абц" too.
So if you need only for english, use
Regex.IsMatch(textBox1.Text, #"^[a-zA-Z0-9#.]*$")

Try this :
Regex.IsMatch(textBox1.Text, #"^[a-zA-Z0-9#.]*$")

Use
^[-a-zA-Z0-9 _ - \. #]*
as the Regex expression text.

Regular expression for DataAnnotation for Password

We have a requirement to make sure password conforms to the specific strength format (this is configured in web.config). Requirements are that password must contain certain number of capital characters and certain number of non-alphanumeric characters. I want to annotate my Password property with regular expression that validates password to make sure password contains x number of CAPS and y number of non-alpha chars. Please help with regular expression.

Checking multiple conditions like this in a single regex is best accomplished using lookaheads, for example say you want 3 capital characters and 4 non-alpha characters, you could use the following regex:
^(?=(?:.*[A-Z]){3})(?=(?:.*[^a-zA-Z]){4})
Explanation: first, lets think about what a regex would look like that only checks the first condition. To match 3 uppercase characters we can use the following:
(?:.*[A-Z]){3}
We can still check this condition by dropping it inside of a lookahead, which is what the (?=...) does, so now (?=(?:.*[A-Z]){3}) checks this condition without consuming any characters. At this point we can check the second condition using (?:.*[^a-zA-Z]){4}. I put this second condition inside of a lookahead as well so that adding more checks is straightforward.
Note that the current regex won't actually match any characters, it will match the beginning of the string (zero characters) if all conditions match, otherwise the match will fail. If you want it to actually consume characters as well, just add .* to the end.

I found a good article that solves it for me
http://www.zorched.net/2009/05/08/password-strength-validation-with-regular-expressions/

Pattern matching for swedish character

I need a help regarding regular expression.
I have to match string like this:
âãa34dc
Pattern that i have used:
\s*[a-zA-Z]+[a-zA-Z_0-9]*\s
but this pattern is not good enough to identify this kind of string e.g. âãa34dc
P.S. âã these are swedish character.
Please help me for find out correct pattern for this kind of string.

Do you actually want to restrict it to Swedish characters? In other words, should a German character not match? If so, then you'll probably have to enumerate the whole alphabet, and include that.
If what you really want is to match every alphabetic character, use the regular expression terms for matching all letters.
\w matches any word character, but that includes numbers & some punctuation. That's close, but not exactly what you want for your second term.
For the first term, where you don't want to include numbers, specifying that the character should be a Unicode 'letter' class will work. \p{L} specifies all Unicode characters that are a letter. This includes [a-zA-Z], and all the Swedish characters, and German, and Russian, etc.
Therefore, I think this regular expression is what you want:
\s*[\p{L}][\p{L}_0-9]*\s
If you want to include digits from other character sets, and some other punctuation, then you can use [\w]* for the second term.

please give a set of rules.
according to your question :
[X-Ya-zA-Z]{3}[0-9]{2}[a-zA-Z]{2}
Replace X with the first swedish letter
Replace Y with the last swedish letter

John Machin provides a great answer for this. Adapting his pattern, what you need is probably something similar to: \s*[^\W\d_]\w*\s*
P.S. I removed the + quantifier from your first part. Any subsequent letters would be matched by the subsequent quantified \w.

How Can I Check If a C# Regular Expression Is Trying to Match 1-(and-only-1)-Character Strings?

Maybe this is a very rare (or even dumb) question, but I do need it in my app.
How can I check if a C# regular expression is trying to match 1-character strings?
That means, I only allow the users to search 1-character strings. If the user is trying to search multi-character strings, an error message will be displaying to the users.
Did I make myself clear?
Thanks.
Peter
P.S.: I saw an answer about calculating the final matched strings' length, but for some unknown reason, the answer is gone.
I thought it for a while, I think calculating the final matched strings length is okay, though it's gonna be kind of slow.
Yet, the original question is very rare and tedious.

a regexp would be .{1}
This will allow any char though. if you only want alpanumeric then you can use [a-z0-9]{1} or shorthand /w{1}
Another option its to limit the number of chars a user can type in an input field. set a maxlength on it.
Yet another option is to save the forms input field to a char and not a string although you may need some handling around this to prevent errors.
Why not use maxlength and save to a char.

You can look for unescaped *, +, {}, ? etc. and count the number of characters (don't forget to flatten the [] as one character).
Basically you have to parse your regex.

Instead of validating the regular expression, which could be complicated, you could apply it only on single characters instead of the whole string.
If this is not possible, you may want to limit the possibilities of regular expression to some certain features. For instance the user can only enter characters to match or characters to exclude. Then you build up the regex in your code.
eg:
ABC matches [ABC]
^ABC matches [^ABC]
A-Z matches [A-Z]
# matches [0-9]
\w matches \w
AB#x-z matches [AB]|[0-9]|[x-z]|\w
which cases do you need to support?
This would be somewhat easy to parse and validate.