Question

On my website, I let the user send tags like this:

tag tagtwo anothertag

I only want to allow them to use single spaces and letters, so I want to remove numbers, dashes, double spaces, etc. So an invalid string would be:

tag       tag2 another-tag

I have the following code to do this, but I don’t know the correct regex to use.

$tags = strtolower($_POST['imgTags']);
$tags = preg_replace("/regex/", "", $tags);
$tagArray = explode(" ", $tags);

What is the correct regEx to do this?
Also, I might want to replace é and ö with e and o.

Answer 1

This will remove anything but letters and spaces:

$tags = preg_replace("/[^a-z ]/i", "", $tags);

Then this will collapse consecutive spaces:

$tags = preg_replace("/ {2,}/", " ", $tags);

If you want to allow other kinds of whitespace characters, but also replace them with single spaces, try this instead:

$tags = preg_replace("/[^a-z\s]/i", "", $tags);
$tags = preg_replace("/\s+/", " ", $tags);

Regarding your last sentence: there is no generic way to this. You will have to add specific rules. However, preg_replace_callback might help you in determining the unmodified letters.