I have a field where user input their age, and I want to do some error checking. If the user enter the digit '0' and click `submit`, it will display an error saying they can't begin with a zero. But I also want to allow an age that contain the digit zero, such as age 20. I don't know how to not allow someone to enter a beginning digit of 0. 

My PHP code is:

```
<?php
$age_numeric = preg_match("/^(^[1-9]){1,}$/", $age);
?>
//some more code
<?php if (!$age_numeric) echo "age can only be numeric."; ?>

```