Regex To Validate A Comma Separated List Of Options

I'm using PHP's Filter Functions (FILTER_VALIDATE_REGEXP specifically) to validate the input data. I have a list of options and the $input variable can specify a number of options

Solution 1:

Sometimes regular expressions just make things more complicated than they should be. Regular expressions are really good at matching patterns, but when you introduce external rules that have dependencies on the number of matched patterns things get complicated fast.

In this case I would just split the list on comma and check the resulting strings against the rules you just described.

$valid_choices = array('checkin','join','promotions','rewards','stream','verified_checkin');

$input_string;                       // string to match$tokens = explode(','$input_string);

$tokens = asort($tokens);            // sort to tokens to make it easy to find duplicatesif($tokens[0] == 'all' && count($tokens) > 1)
    returnFALSE;                    // fail (all + other options)if(!in_array($tokens[0], $valid_choices))
    returnFALSE;                    // fail (invalid first choice)for($i = 1; $i < count($tokens); $i++)
{
    if($tokens[$i] == $tokens[$i-1])
       returnFALSE;                 // fail (duplicates)if(!in_array($tokens[$i], $valid_choices))
       returnFALSE;                 // fail (choice not valid)
}

EDIT

Since you edited your and specified that duplicates would be acceptable but you definitely want a regex-based solution then this one should do:

^(all|((checkin|verified_checkin|rewards|join|promotions|stream)(,(checkin|verified_checkin|rewards|join|promotion|stream))*))$

It will not fail on duplicates but it will take care or leading or trailing commas, or all + other choices combination.

Filtering out duplicates with a regex would be pretty difficult but maybe not impossible (if you use a look-ahead with a capture group placeholder)

SECOND EDIT

Although you mentioned that detecting duplicate entries is not critical I figured I'd try my hand at crafting a pattern that would also check for duplicate entries.

As you can see below, it's not very elegant, nor is it easily scalable but it does get the job done with the finite list of options you have using negative look-ahead.

^(all|(checkin|verified_checkin|rewards|join|promotions|stream)(,(?!\2)(checkin|verified_checkin|rewards|join|promotions|stream))?(,(?!\2)(?!\4)(checkin|verified_checkin|rewards|join|promotions|stream))?(,(?!\2)(?!\4)(?!\6)(checkin|verified_checkin|rewards|join|promotions|stream))?(,(?!\2)(?!\4)(?!\6)(?!\8)(checkin|verified_checkin|rewards|join|promotions|stream))?(,(?!\2)(?!\4)(?!\6)(?!\8)(?!\10)(checkin|verified_checkin|rewards|join|promotions|stream))?)$

Since the final regex is so long, I'm going to break it up into parts for the sake of making it easier to follow the general idea:

^(all|
  (checkin|verified_checkin|rewards|join|promotions|stream)
  (,(?!\2)(checkin|verified_checkin|rewards|join|promotions|stream))?
  (,(?!\2)(?!\4)(checkin|verified_checkin|rewards|join|promotions|stream))?
  (,(?!\2)(?!\4)(?!\6)(checkin|verified_checkin|rewards|join|promotions|stream))?
  (,(?!\2)(?!\4)(?!\6)(?!\8)(checkin|verified_checkin|rewards|join|promotions|stream))?
  (,(?!\2)(?!\4)(?!\6)(?!\8)(?!\10)(checkin|verified_checkin|rewards|join|promotions|stream))?
 )$/

You can see that the mechanism to form the pattern is somewhat iterative and such a pattern could be generated automatically by an algorithm if you wanted to provide a different list but the resulting pattern would get rather large, rather quickly.