How to retrieve the opacity value within a mixin - css

Question

So I have the following problem, I want to be able to select a row in a table, and when I do that row gets an active background color. So far no problems. However, if the user should specify the background color to be transparent or set the opacity in rgba()-string to 0 then I want to display the underlaying row-color.
Now if I use a mixing I could style my css in different ways. But how can I get the opacity from the rgba-string to determine whether the string should be transparent or not? Does regex work within a mixin?
I.e.: I would something like (I know, the regex part is js but somthing similar for css/mixin):
#mixin active-background-color($activeBackgroundColor){
#if ($activeBackgroundColor.replace(/^.*,(.+)\)/,'$1') == 0) {
background-color: none;
} #else {
background-color: $activeBackgroundColor
}
}
So the solution turned out to be the following (should anyone be interested):
#mixin active-background-color($color) {
$alphaVal: alpha($color);
#if ($alphaVal== 0){
background-color: inherit;
} #else {
background-color: $color;
}
}
Link to the solution I used: The Sass Way

Answer 1

Sounds like you should simply define the selection on the tr
and add the user picked color to the td:
tr { background-color: whitesmoke; } // default row color
tr:hover { background-color: #ccc; } // default row selected color
tr:hover td { background-color: tomato } // user defined selected color
// if the user selects a transparent selection color
// the default row selected color will show
table { width:100%; border-collapse: collapse;}
tr { cursor: pointer; }
td { font:1rem sans-serif; padding:.2rem; }
tr { background-color: whitesmoke; }
tr:hover { background-color: #ccc; }
tr:hover td { background-color: tomato }
tr:last-of-type:hover td { background-color: transparent }
<table>
<tr><td>One</td><td></td><td></td><td></td></tr>
<tr><td>Two</td><td></td><td></td><td></td></tr>
<tr><td>Three</td><td></td><td></td><td></td></tr>
</table>