Sorry - as others already pointed out - your question is imprecise.

What exactly does "missing" or "90%" mean?

Have single letters been

• deleted,
• replaced or
• inserted

to one of the strings?

Does order matter?

What's the alphabet and are all characters equally interchangeable?

How is the match x% calculated?

Please show a concise example for all cases and give us estimates about the normal size of the data in question.

And I'll promise you an equally good answer...

HTH! :)