The pattern provided to PHP regular expression functions is required to be enclosed in valid delimiters. Failing to do so will result in a PHP warning and the pattern never matching. Since the warning only appears during runtime when the pattern is evaluated, such a mistake risks to get unnoticed into production.

A delimiter can be any character that is not alphanumeric, a backslash, or a whitespace. Bracket style delimiters are also allowed.

Noncompliant Code Example

// Condition will always evaluate to false
if (preg_match("/.*", $input)) {
    echo "true";
} else {
    echo "false";
}

// Unclosed bracket delimiters
$result = preg_match("[abc", $input);

Compliant Solution

if (preg_match("/.*/", $input)) {
    echo "true";
} else {
    echo "false";
}

$result = preg_match("[abc]", $input);

See

• Delimiters - PHP Documentation