Regex to match a whole string only if it lacks a given substring/suffix

Question

I\'ve searched for questions like this, but all the cases I found were solved in a problem-specific manner, like using !g in vi to negate the regex matches,

Answer 1

Try this regular expression:

^(?:(?!Foo).)*$

This will consume one character at a time and test if there is no Foo ahead. The same can be done with a negative look-behind:

^(?:.(?<!Foo))*$

But you can also do the same without look-around assertions:

^(?:[^F]*|F(?:$|[^o].|o(?:$|[^o])))*$

This matches any character except F or an F that is either not followed by a o or if followed by an o not followed by another o.