# man > Regexp::Common::_support(3pm)

---
type: CommandReference
command: Regexp::Common::support
mode: perldoc
section: 3pm
source: perldoc
---

## Quick Reference

- `luhn($number)` — returns true if the argument passes the Luhn checksum test

## Name

`Regexp::Common::support` -- Support functions for `Regexp::Common`.

## Synopsis

perl
use Regexp::Common::_support qw /luhn/;

luhn ($number);    # Returns true/false.
## Options

- `luhn($number)` -- Returns true if its argument passes the Luhn checksum test. This subroutine is intended for use by other `Regexp::Common` modules and is not meant to be used directly. Its interface may change without notice.

## See Also

- [Regexp::Common](http://localhost/phpMan.php/perldoc/Regexp%3A%3ACommon/markdown)
- [Luhn formula](http://www.webopedia.com/TERM/L/Luhn_formula.html)