Zipcode validation for Gravity Forms (5 digits, 2 approaches).

zipcode-2.php

<?php

# just add the needed form IDs to the array
$forms = array( '6', '7' );

# looping through the array to add an 'add_filter' for each
foreach ( $forms as $i => $form )
    add_filter( "gform_field_validation_{$form}", 'custom_zip_validation', 10, 4 );

# the function
function custom_zip_validation( $result, $value, $form, $field )
{
	if ( ( 'address' == $field->type ) && $result['is_valid'] )
	{	
		$zip_value = rgar( $value, $field->id . '.5' );

		if ( ! ctype_digit( $zip_value ) || 5 != strlen( $zip_value ) ) 
		{
			$result['is_valid'] = false;
			$result['message'] = 'Please check your Zip Code. It must be 5 digits only.';
		}
	}
	
	return $result;
} // end custom_zip_validation

zipcode-3.php

<?php

# the filter, change 7 to the actual form ID
add_filter( 'gform_field_validation_7', 'custom_zip_validation', 10, 4 );

# the function
function custom_zip_validation( $result, $value, $form, $field )
{
	if ( ( 'address' == $field->type ) && $result['is_valid'] )
	{	
		$zip_value = rgar( $value, $field->id . '.5' );
		if ( ! ctype_digit( $zip_value ) || 5 != strlen( $zip_value ) ) 
		{
			$result['is_valid'] = false;
			$result['message'] = 'Please check your Zip Code. It must be 5 digits only.';
		}
	}
	
	return $result;
} // end custom_zip_validation

zipcode.php

<?php

# each form ( + field ID) its own 'add_filter'
add_filter( 'gform_field_validation_6_15', 'custom_zip_validation', 10, 4 );
add_filter( 'gform_field_validation_7_15', 'custom_zip_validation', 10, 4 );

# the function
function custom_zip_validation( $result, $value, $form, $field )
{
	if ( $result['is_valid'] )
	{	
		$zip_value = rgar( $value, $field->id . '.5' );

		if ( ! ctype_digit( $zip_value ) || 5 != strlen( $zip_value ) ) 
		{
			$result['is_valid'] = false;
			$result['message'] = 'Please check your Zip Code. It must be 5 digits only.';
		}
	}
	
	return $result;
} // end custom_zip_validation

https://community.gravityforms.com/t/zip-code-field-validation/869/8