Search keywords contain numbers

If I search for Milk 1.5 Liter, how can I show the results Milk 1.5L before Milk 2.5L

I think there's a number of ways you could address this, but would probably depend on your exact use case.

If you could separate 1.5 and 2.5 into a distinct field, like "measurement" or something, then you could probably apply a higher Weight to that field to ensure that matches on that field factor more heavily in scoring.