PHP extend regex to accept round brackets - php

Question

I have an existing regex which checks if the string is not wrapped in either quotes or square brackets then
I'm wrapping that string in quotes
My existing regex is as follow -
if (!preg_match('/^["\[].*["\]]$/', $filter)) {
$filter = '%22' . $filter . '%22';
}
Now I want to extend this regex to check already wrapped in either quotes or square brackets or parentheses
For parentheses, my string value i.e my $filter value would be something like (123 456)
Can anyone help to extended this regex?

Answer 1

I think the regular expression is good for complex string math but like this simple string match why do you use regex it takes almost O(n) time where n is you string size some time it takes O(2^m), where m is the length of regex. But if you check with a simple if just check 1st and the last characters of string it takes O(1). Here is the regex solution.
/^(?(?=")(["].*["])|(?=\[)([\[].*[\]]))|(?=\()(([\(].*[\)]))$/

Answer 2

The regular expressions are a powerful tool but they are not a Swiss army knife. There are problems that simply cannot be resolved using regex and there are problems that can be resolved using regex but a simpler approach produces code that is easier to read and understand. This is such a problem.
Let's reformulate the problem. If the first character of the string is " and also its last character is " then the string is wrapped in quotes and it does not need other processing. The same when the first character is [ and the last one is ]. Or ( and ).
The first character of a string stored in the variable $filter is $filter[0]. The last one is $filter[-1]. Extract them into a new string and search for it into a list of combinations of quotes and parentheses:
if (! in_array($filter[0].$filter[-1], ['""', '[]', '()'])) {
// the string is not enclosed in quotes, square brackets or parentheses
// do something with it (enclose it, etc)
}
If you are using PHP 5 (any version) or PHP 7.0 then you are out of luck (and out of PHP updates, btw) and you cannot use $filter[-1] (because this functionality has been introduced in PHP 7.1).
The PHP function substr() comes to the rescue.
substr($filter, -1) does the same thing as $filter[-1] (returns the last character of $filter and works in all PHP versions.
There are two corner cases to consider:
When $filter is '"' (a string of exactly one character that is a double quote), the code above will report it as enclosed in quotes when, in fact, it is not.
When $filter is '' (the empty string) the code produces two warnings (but does not report it as being enclosed in quotes.
Both cases can be easily solved by adding a check of the string's length to avoid running the other test if the string is too short:
if (strlen($filter) < 2 || ! in_array($filter[0].$filter[-1], ['""', '[]', '()'])) {
// the string is not enclosed in quotes, square brackets or parentheses
// do something with it (enclose it, etc)
}