regular expression

I am trying to validate a fax number. The fax number is optional. The field can be blank OR it can be a valid 10 digit phone number. I tried using the regular expression option.

$form->AddInput(array(
"TYPE"=>"text",
"NAME"=>"fax",
"ID"=>"fax",
"MAXLENGTH"=>12,
"LABEL"=>"<u>F</u>ax",
"ACCESSKEY"=>"F",
"TABINDEX"=>$ti++,
"ValidateRegularExpression"=>array(
'^$',
'[0-9]{3}-[0-9]{3}-[0-9]{4}'),
"ValidateRegularExpressionErrorMessage"=>array(
'',
"Please use a 10 digit fax number seperated by dashes.")
));

How can I make something valid if is blank OR matches the regular expression.

I am not sure if regular expressions can match empty values. Anyway, you can always use the ValidateOptionalValue parameter set to an empty string to make the validation accept empty strings without further validation.

Thanks. I set ValidateOptionalValue to '' and the field now accepts blank values OR valid 10 digit fax numbers. Here is the code for those interested

$form->AddInput(array(
"TYPE"=>"text",
"NAME"=>"fax",
"ID"=>"fax",
"MAXLENGTH"=>12,
"LABEL"=>"<u>F</u>ax",
"ACCESSKEY"=>"F",
"TABINDEX"=>$ti++,
"ValidateOptionalValue"=>'',
"ValidateRegularExpression"=>array('[0-9]{3}-[0-9]{3}-[0-9]{4}'),
"ValidateRegularExpressionErrorMessage"=>array("Please use a 10 digit fax number separated by dashes."),
));