Why Does My `preg_match()` Fail After a PHP Upgrade?

preg_match() Compilation Failure in Character Class Range Due to PHP Upgrade

The error "preg_match(): Compilation failed: invalid range in character class at offset" often occurs when using PHP's preg_match function to validate alphanumeric usernames. This error has been reported to manifest after PHP upgrades on servers.

The issue stems from recent changes introduced in PHP 7.3 and newer versions. PHP has migrated its PCRE engine to PCRE2, which brings about stricter pattern validation. Specifically, it concerns the use of hyphens (-) in character classes.

In PHP versions prior to 7.3, it was possible to use hyphens in character classes by escaping them or placing them at the beginning or end of the range. However, in PHP 7.3 and later, the PCRE2 engine has tightened this requirement.

The error occurs because the hyphen in the character class [a-z0-9]([0-9a-z_-s]) $ is not at the beginning or end of the range. This leads to the compilation failing.

To resolve the issue, you should modify the character class as follows:

/[a-z0-9]([0-9a-z-_ ]+)+/i

By placing the hyphen at the end of the range, it will be interpreted as a literal character within the class, rather than indicating a range.

It's important to note that other pattern syntax changes may have been introduced with the update to PHP 7.3. For more information on these changes and how they might affect your code, please refer to the official PHP documentation on PCRE2 migration.

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