title: regular expression to blacklist potentially dangerous strings
body: <p>What regular expression should be used to blacklist the character sequence that would cause the following error in ASP.Net - "A potentially dangerous Request.Form value was detected from the client". (e.g. &lt; br >, &lt; hi >, &lt; a > etc..) </p>
  
  <p>I need to use a <strong>asp.net regular expression validation control</strong> to <strong>blacklist</strong> those character sequence that would cause this error.</p>
  
  <p>for instance if I enter 123&lt; hi > , valid &lt; abc >, etc.. i need the validation to fail.</p>
  
  <p>PS: I have spaced the &lt; br > since it was not displaying it properly in the question but actually it should be without the blank spaces between &lt; or > and the string inbetween them</p>