form validation with preg_match - forms

Question

I have created a form which gets user data about the city he leaves. The form stores data in a table called profile. I am using a function which inserts data into profile table. If there are already data, the function just updates the table. If the user does not insert data into city field and press submit, the table continues to keep the previous data, because nothing submitted.
In the form I would like to make a validation so that the user will inserts only a-z or space characters.
I have used the following code for validation:
if( empty($_POST) === false ){
if( empty($errors) === true ){
if( preg_match('/^[a-zA-Z ]+$/', $_POST['city']) ===0 ){
$errors[]='<font color="#963">City must only contains a-z and spaces</font>';
}
}
}
It works! But the problem now is that if the user leave the form empty and press submit the above code enforces him to place data( says: City must only contains a-z and spaces). How can I use the above code to validate the form only if the user enters data. And if not then to continue without any problem.

Answer 1

You can do this without even generating an error. Replace
if( preg_match('/^[a-zA-Z ]+$/', $_POST['city']) ===0 ){
With
$_POST['city'] = preg_replace("&[^a-zA-Z ]&","",$_POST['city']);
This will replace all non-lowercase letters and non-spaces with nothing before it's inserted into the database.
EDIT:
If you'd still like to generate an error, use
if( preg_match("&[^a-zA-Z ]&",$_POST['city']) ){
$errors[]='<font color="#963">City must only contains a-z and spaces</font>';
}
before the preg_replace.