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# Pragmatic Regular Expression Exclude

Sometimes you cannot simply write:

`if str.match(regexp) and not str.match(other_regexp) do_something`

and you have to make this behaviour with only one regular expression. But, there exists a major problem: the complementary of a regular language might not be regular. Then, for some expression it is absolutely impossible to negate a regular expression.

But sometimes with some simple regular expression it should be possible†. Say you want to match everything containing the some word say `bull` but don’t want to match `bullshit`. Here is a nice way to do that:

`# match all string containing ‘bull’ (bullshit comprised) /bull/`

# match all string containing ‘bull’ except ‘bullshit’

/bull([^s]|)|b u l l s([h]|)| bullsh([^i]|)|b u l l s h i([t]|)/

# another way to write it would be

/bull([^s]||s([h]|)|sh([^i]|)|s h i([t]|))/

Let look closer. In the first line the expression is: `bull([^s]|$)`, why does the `$` is needed? Because, without it the word `bull` would be no more matched. This expression means:

> The string finish by `bull`
> or,
> contains `bull` followed by a letter different from `s`.

And this is it. I hope it could help you.

Notice this method is not always the best. For example try to write a regular expression equivalent to the following conditional expression:
`# Begin with ‘a’: ^a # End with ‘a’: c$ # Contain ‘b’: .b. # But isn’t ‘axbxc’ if str.match(/^a.b.c/)andnotstr.match(/axbxc/) do_something end`

A nice solution is:

`/abc| # length 3 a.bc| # length 4 ab.c| a[^x]b[^x]c| # length 5 a…b.c| # length >5 a.b…c/`

This solution uses the maximal length of the string not to be matched. There certainly exists many other methods. But the important lesson is it is not straightforward to exclude something of a regular expression.

† It can be proved that any regular set minus a finite set is also regular.

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Published on 2010-02-15
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Yann Esposito©
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