How can I get a numeric representation of a utf8 encoded string?

Essentially, you cannot for strings of any usable length.

Unicode strings stored in computers are essentially very big numbers stored base-131072 (if you stick to just the very basic multi-lingual subset).

With decimal numbers encoded as a string (eg. '12345'), each digit can be 0-9, so the size of the number grows by a factor of 10 for each extra digit. So by the time you've got 20 digits, you've exhausted the capacity of 64-bit integers. And if you move to floating point, you start loosing accuracy after just 15 digits.

For hex numbers encoded as a string, each extra digit adds a factor of 16, so you exhaust 64-bits ints with only 16 digits.

With Unicode you have 128k for each digit, so by the time you've got a 4 character string you've exceeded the capacity of a 64-int by a factor of 10.

So basically, you can give up on the idea of an accurate representation of a string by a number.

Then you move into the realm of 'lossy' representations. And there is a whole science (and a lot of bunkum) that attempts to produce 'comparative' numerical values from documents -- numbers derived from text that will when sorted numerically tend to group the documents by similarity. These are used for applications such as plagiarism detection. A simple starting point which may lead you in many directions.

Another approach might be to use a 'running checksum' to detect sections of similarity. For that approach the Rsynch algorithm is a useful starting point.

For the most part, if your goal is simply to save space in your db, you'd probably be better off using a simple compression algorithm.

Update: for completeness, you might find Re^3: Comparing sets of phrases stored in a database? enlightening.