Performing find and replace in QGIS field calculator?

I'm trying to replace selected string values in an attribute table, eg. "Text1" for "Text2" (QGIS 1.8.0), patriculary for those rows with NULL values. I have been trying to use the replace expression (and any other that I have found) with no luck.

Are there any wildcard characters that I may be able to use in future?

Answer

You could try a case when expression:

case when "Column_1" is NULL then  replace("Column_2",'Text2','Text1') else "Column_2" end

Using the 'or' expression, you can add multiple columns with NULL-values to the code.

case when "Column_1" is NULL or "Column_3" is NULL then  replace("Column_2",'Text2','Text1') else "Column_2" end

edit:

If you want to replace NULL, forget the replace-expression (it only works for strings and NULL is no string). Try this exact code:

case when "Feature" is NULL then '12_1' else "Feature" end