When an alternation contains multiple alternatives that consist of a single character, it can be rewritten as a character class. This should be preferred because it is more efficient and can even help prevent stack overflows when used inside a repetition (see rule {rule:php:S5998}).

Noncompliant Code Example

preg_match("/a|b|c/", $str); // Noncompliant

Compliant Solution

preg_match("/[abc]/", $str);
// or
preg_match("/[a-c]/", $str);