How to exclude a specific string constant? [duplicate]

Can regular expression be utilized to match any string except a specific string constant (i.e. "ABC")?

Is it possible to exclude just one specific string constant?

You have to use a negative lookahead assertion.

(?!^ABC$)

You could for example use the following.

(?!^ABC$)(^.*$)

If this does not work in your editor, try this. It is tested to work in ruby and javascript:

^((?!ABC).)*$

This isn't easy, unless your regexp engine has special support for it. The easiest way would be to use a negative-match option, for example:

$var !~ /^foo$/
    or die "too much foo";

If not, you have to do something evil:

$var =~ /^(($)|([^f].*)|(f[^o].*)|(fo[^o].*)|(foo.+))$/
    or die "too much foo";

That one basically says "if it starts with non-f, the rest can be anything; if it starts with f, non-o, the rest can be anything; otherwise, if it starts fo, the next character had better not be another o".

In .NET you can use grouping to your advantage like this:

http://regexhero.net/tester/?id=65b32601-2326-4ece-912b-6dcefd883f31

You'll notice that:

(ABC)|(.)

Will grab everything except ABC in the 2nd group. Parenthesis surround each group. So (ABC) is group 1 and (.) is group 2.

So you just grab the 2nd group like this in a replace:

$2

Or in .NET look at the Groups collection inside the Regex class for a little more control.

You should be able to do something similar in most other regex implementations as well.

UPDATE: I found a much faster way to do this here: http://regexhero.net/tester/?id=997ce4a2-878c-41f2-9d28-34e0c5080e03

It still uses grouping (I can't find a way that doesn't use grouping). But this method is over 10X faster than the first.

You could use negative lookahead, or something like this:

^([^A]|A([^B]|B([^C]|$)|$)|$).*$

Maybe it could be simplified a bit.

Try this regular expression:

^(.{0,2}|([^A]..|A[^B].|AB[^C])|.{4,})$

It describes three cases:

1. less than three arbitrary character
2. exactly three characters, while either
   • the first is not A, or
   • the first is A but the second is not B, or
   • the first is A, the second B but the third is not C
3. more than three arbitrary characters