You use the "zero or more" * operator, which will match empty strings in front of the "%". But an empty string cannot be converted to a float. Just use the "at

You use the "zero or more" * operator, which will match empty strings in front of the "%". But an empty string cannot be converted to a float. Just use the "at least one" operator: +. Also, the scale is 0,100 by default, you can omit it. df -h | colout "([0-9]+)%" scale