validation - JavaScript - how to validate field to allow certain character and certain length -

i need validate textbox, it's value consist of 10 characters (not more, not less). below code it, allowing me set restricted length each field separately.

 function charlength(elem, min, max){  var uinput = elem.value;  if(uinput.length >= min && uinput.length <= max){                 alert("invalid number of characters");   elem.focus();   return false;  }else{   return true;  } } 

and how i'm calling it:

onblur="charlength(document.getelementbyid('tf1'),1,9)" 

but field validate must not 10 characters long, has start letter z.

how such validation? possible in first place?

try also:

 function charz10(elem, min, max){  var uinput = elem.value;      if(uinput.length >= min && uinput.length <= max && uinput.substr(0, 1) == "z"){           alert("material number has consist of 10 characters begining z");           elem.focus();         return false;      }      else return true; //i  } 

and try add maxlength in textbox

<input type="text" maxlength="10">