Regex for hex colours: three, six and eight digit forms

The preset is compact because it repeats a group of three rather than listing two lengths: one iteration gives the shorthand form and two give the full one. That construction is exact, so it never matches four or five digits, and the trailing word boundary is what makes it exact rather than merely greedy.

Removing that boundary would break it quietly. Against #abcd the group would fall back to one iteration, match #abc, and report success on a string that is not a valid colour in either form. With the boundary in place the fallback is rejected too, because the d that follows is a word character and there is no boundary between it and the c.

The gap is the alpha forms. CSS accepts four and eight digit colours, where the extra digits carry opacity, and the preset matches neither. A pattern that covers all four lengths has to list them longest first, because an alternation is ordered and a shorter branch listed earlier would match a prefix and stop.

As always, the pattern checks shape and not meaning. It says nothing about whether the colour is readable against its background, and a validator built on it should still be paired with a real parse if the value is going anywhere near a rendering pipeline.