Discovered by Yeting Li, c/o Colin Ife via Snyk.io.

A ReDos (Regular Expression Denial of Service) vulnerability
was responsibly disclosed to me via email by Colin on
Mar 5 2021 regarding an exponential time complexity for
linearly increasing input lengths for `hwb()` color strings.

Strings reaching more than 5000 characters would see several
milliseconds of processing time; strings reaching more than
50,000 characters began seeing 1500ms (1.5s) of processing time.

The cause was due to a the regular expression that parses
hwb() strings - specifically, the hue value - where
the integer portion of the hue value used a 0-or-more quantifier
shortly thereafter followed by a 1-or-more quantifier.

This caused excessive backtracking and a cartesian scan,
resulting in exponential time complexity given a linear
increase in input length.

Thank you Yeting Li and Colin Ife for bringing this to my
attention in a secure, responsible and professional manner.

A CVE will not be assigned for this vulnerability.