Regex to add hyphens at specific location of string

I found this regex which works fine if there are no alphabets in provided string.

$string = "12522191813381147500333332228323";
echo $formattedString = preg_replace("/^(\d{8})(\d{4})(\d{4})(\d{4})(\d{12})$/", "$1-$2-$3-$4-$5", $string);

My input string would be sometime mix of alphabets and numbers. What changes I have to do to make it work i开发者_开发问答n both the cases. I have other alternative to iterate a string and add dashes using string functions of PHP but I want to learn how we can achieve it using regular expression.

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\w also allows underscores, if you really only want to allow alphanumeric characters, you'd have to do something like this:

([a-z0-9]{8})([a-z0-9]{4})

And include a i flag at the end of the regex (after the final /) to make it case insensitive.