You're not alone. I consider my Fu to be fair-to-middlin', and conditionals just make my head hurt. I've yet to find a use for them, myself. Lookarounds usually suffice for my needs. For instance, the following expression should ignore any form of "font-size: 1em". Regardless of whether it's:
font-size: 1em
font-size:1em
font-size: 1.0em
font-size:1.0em
font-size: 1.00em
font-size: 1em;
Code:
font-size:(?!\s*(1(\.0+)?em))\s*\K[^(;|})]*
You should then be able to replace the resulting match with any value you like.
I realize the regex conditional might have become your "white whale" at this point. If so, feel free to share any working solution you might eventually come up with.
