How to: Strip Invalid Characters from a String

String CleanInput(string strIn)
    {
        // Replace invalid characters with empty strings.
        return Regex.Replace(strIn, @"[^\w\.@-]", "");
    }

The regular expression "^[0-9]*$" only matches strings that consist entirely of digits.


Othe useful expressions:
{n,} - matches at leas n times
{n,}? - nongreedy {n,} quantifier
\s - only spaces

Allowing only single space in between two words

([a-zA-Z]{1}[a-zA-Z]*[\s]{0,1}[a-zA-Z])+([\s]{0,1}[a-zA-Z]+)

This Regular expression will simply not allow more than a single space to be entered between two words of a sentence.It will also prohibit user to enter space at the begining and at the end of a sentence.It will require a minimum of 2 alphabets for making it a valid expression.

For any words+digits:

([a-zA-Z0-9]{1}[a-zA-Z0-9]*[\s]{0,1}[a-zA-Z0-9])+([\s]{0,1}[a-zA-Z0-9]{2,})

The 30 Minute Regex Tutorial

In Russian language

regexlib.com/DisplayPatterns.aspx