Question

Suppose I want to match all strings except one: “ABC”
How can I do this?

I need this for a regular expression model validation in asp.net mvc 3.

Answer 1

Normally you would do like

(?!ABC)

So for example:

^(?!ABC$).*

All the strings that aren’t ABC

Decomposed it means:

^ beginning of the string
(?!ABC$) not ABC followed by end-of-string
.* all the characters of the string (not necessary to terminate it with $ because it is an eager quantifier)

Technically you could do something like

^.*(?<!^ABC)$

Decomposed it means

^ beginning of the string
.* all the characters of the string 
(?<!^ABC) last three characters captured aren't beginning-of-the-string and ABC 
$ end of the string (necessary otherwise the Regex could capture `AB` of `ABC` and be successfull)

using a negative look behind, but it is more complex to read (and to write)

Ah and clearly not all the regex implementations implement them 🙂 .NET one does.