Input Fields With Most Important Cases?

we have form of the following code

Code:

<form name="name" method="post" action="index.html">
<input type="text" name="1" size="40" maxsize="6">
<input type="text" name="2" size="40">
</form>

I want someone give me an correct example for the following cases

for 1st one

Code:

<input type="text" name="1" size="40" maxsize="6">

1- Make it accept only numbers
2- Make it accept only numbers and only 6 digits if less then gives error

for 2nd one

Code:

<input type="text" name="2" size="40">

1- Accept only letters with no numbers

for both 1 and 2

1- Can't write HTML codes within it