Question

I use the following regular expression to validate a comma separated list of values.

^Dog|Cat|Bird|Mouse(, (Dog|Cat|Bird|Mouse))*$

The values are also listed in a drop down list in Excel cell validation, so the user can select a single value from the drop down list, or type in multiple values separated by commas.

The regular expression does a good job of preventing the user from entering anything but the approved values, but it doesn’t prevent the user from entering duplicates. For example, the user can enter “Dog” and “Dog, Cat”, but the user can also enter “Dog, Dog”.

Is there any way to prevent duplicates using a similar single regular expression? In other words I need to be able to enforce a discrete list of approved comma separated values.

Thanks!

Answer 1

Use a backreference and a negative lookahead:

^(Dog|Cat|Bird|Mouse)(, (?!\1)(Dog|Cat|Bird|Mouse))*$

EDIT: This won’t work with cases such as “Cat, Dog, Dog” … You’ll need to come up a hybrid solution for such instances – I don’t believe there is a single regex that can handle that.

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Here’s another technique. You need to check two things, first, that it DOES match this:

(?:(?:^|, )(Dog|Cat|Bird|Mouse))+$

(That’s just a slightly shorter version of your original regex)

Then, check that it DOES NOT match this:

(Dog|Cat|Bird|Mouse).+?\1

E.g.

var valid = string.match( /(?:(?:^|, )(Dog|Cat|Bird|Mouse))+$/ ) &&
           !string.match( /(Dog|Cat|Bird|Mouse).+?\1/ );