Why is this an issue?

Overly complicated regular expressions are hard to read and to maintain and can easily cause hard-to-find bugs. If a regex is too complicated, you should consider replacing it or parts of it with regular code or splitting it apart into multiple patterns at least.

The complexity of a regular expression is determined as follows:

Each of the following operators increases the complexity by an amount equal to the current nesting level and also increases the current nesting level by one for its arguments:

• | - when multiple | operators are used together, the subsequent ones only increase the complexity by 1
• Quantifiers (*, +, ?, {n,m}, {n,} or {n})
• Non-capturing groups that set flags (such as (?i:some_pattern) or (?i)some_pattern)
• Lookahead and lookbehind assertions

Additionally, each use of the following features increase the complexity by 1 regardless of nesting:

• character classes
• back references

Noncompliant code example

if (preg_match("/^(?:(?:31(\\/|-|\\.)(?:0?[13578]|1[02]))\\1|(?:(?:29|30)(\\/|-|\\.)(?:0?[13-9]|1[0-2])\\2))(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$|^(?:29(\\/|-|\\.)0?2\\3(?:(?:(?:1[6-9]|[2-9]\\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))$|^(?:0?[1-9]|1\\d|2[0-8])(\\/|-|\\.)(?:(?:0?[1-9])|(?:1[0-2]))\\4(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$/", $dateString)) {
    handleDate($dateString);
}

Compliant solution

    if (preg_match("/^\\d{1,2}([-/.])\\d{1,2}\\1\\d{1,4}$/"), $dateString) {
        $dateParts = preg_split("/[-/.]/", $dateString);
        $day = intval($dateParts[0]);
        $month = intval($dateParts[1]);
        $year = intval($dateParts[2]);
        // Put logic to validate and process the date based on its integer parts here
    }