Negate a part of regular expression

Sometimes it is useful to negate a part of a regular expression. Not in all cases negating a group with the ^ character gets the job done.

Often negating a particular string is required. Technically what is usually needed then is called a negative lookahead.

When for example we want to match the string "AB" not followed by "CD" then the regular expression should be:

AB(?!CD)

The above will match "ABDC" but will not match "ABCD".