I would not want to appear the labels like 'xxxxxx_FIUME_xxxxx' or 'FIUME_xxxxx' etc. I tried using the rue-based option with an expression like "NOME" NOT LIKE '% [_]FIUME%' but without success (all features are found). How can i solve the problem?
If you choose rulebased labeling, you can use for example the following expression as a filter to rule out all labels containing "fiume"
This gives you all rivers not containing the string "fiume", then you simply tell QGis how to label these. Then you also have to "catch" all the rivers containing the "fiume"-string
And now tell QGis to not to label these, or maybe label with the same shortened expression for all small rivers.