From: "david v"

Yes i think there were some misunderstanding here.. I think that comes by the definition you have of mistake. A mistake for me is as follows:

Regex: "testing" String_to_search: "tastung". The output should be that the regex testing was found but with 2 mismatches that are "a" and "u". So a mismatch is a letter that was not found.

It may sound weird to you but the way i'm using the regex is to identify genomic regions, so in other words for biological applications. In some cases my regex is a piece of DNA such as "atgcta" and i want to search for this regex in another piece of DNA. Given the fact that the regex "atgcta" can be found in the genome many times i will get probably get a lot of matches. But in some cases i want to be able to search for "atgcta" but i want to allow some mismatches. Obviuously i will even get more matches but i think regex can be a more much efficient way that by building ip aligment matrices.

Any idea how to handle the example above

I don't see how regex can help you here. You need to use more complicated string algorithms. Try this: http://en.wikipedia.org/wiki/Suffix_tree - James