Regular expression that doesn't contain certain string [duplicate]

Question

I have something like this

aabbabcaabda

for selecting minimal group wrapped by a I have this /a([^a]*)a/ which works just fine

But i have problem with groups wrapped by aa, where I'd need something like /aa([^aa]*)aa/ which doesn't work, and I can't use the first one like /aa([^a]*)aa/, because it would end on first occurence of a, which I don't want.

Generally, is there any way, how to say not contains string in the same way that I can say not contains character with [^a]?

Simply said, I need aa followed by any character except sequence aa and then ends with aa

Answer 1

By the power of Google I found a blogpost from 2007 which gives the following regex that matches string which don't contains a certain substring:

^((?!my string).)*$ 

It works as follows: it looks for zero or more (*) characters (.) which do not begin (?! - negative lookahead) your string and it stipulates that the entire string must be made up of such characters (by using the ^ and $ anchors). Or to put it an other way:

The entire string must be made up of characters which do not begin a given string, which means that the string doesn't contain the given substring.

Answer 2

In general it's a pain to write a regular expression not containing a particular string. We had to do this for models of computation - you take an NFA, which is easy enough to define, and then reduce it to a regular expression. The expression for things not containing "cat" was about 80 characters long.

Edit: I just finished and yes, it's:

aa([^a] | a[^a])aa 

Here is a very brief tutorial. I found some great ones before, but I can't see them anymore.