Dyanamically set size of textbox using js

I have to define a textbox in html whose maximum size should be 3.

Now If I am entring -100 then these are 4 characters but I want that user should allow to enter 100 or -100. If I define size="3" then it will not allow -100. So I want to know can we 开发者_StackOverflowdefine size of the textbox dynamically,i.e. if I enter "-" sign then size will increase to 4 else it should be 3.

Can we do this using js?

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Call this function on the key down event of the text box

function check(e){

  var unicode=e.charCode? e.charCode : e.keyCode

  if(if unicode==109){
   //increase the size of text box to 4;
  }

}

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You could use regular expressions in a keydown event: If the regexp doesn't match, the function returns FALSE.

Possible regular expression:

^[-]{0,1}\d{0,3}$