RegEx to exclude a specific string constant

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Regular expression to match a line that doesn't contain a word? 25 answers

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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).)*$

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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 ".

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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.

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