When trying to use "Expression editor" in the guided rule screen of the web app, combining with restrictions on String fields which use functions like "toLowerCase" , "toUpperCase" -
Expression editor leaves a blanc field to fill and if left blanc - KIE automatically fill it with "" - and rule doesn't get compiled.

So instead of getting for example;
1. | package ido.droolsrules.dqsrulesqa;
2. |
3. | import java.lang.Number;
4. |
5. | rule "Test Camera Model"
6. | dialect "mvel"
7. | when
8. | cam1 : Camera( brand.toLowerCase() == "sony" )
9. | then
10. | cam1.setLens( 5.0 );
11. | end

Rule is generated (In source tab) as:

1. | package ebay.droolsrules.dqsrulesqa;
2. |
3. | import java.lang.Number;
4. |
5. | rule "Test Camera Model"
6. | dialect "mvel"
7. | when
8. | cam1 : Camera( brand.toLowerCase("") == "sony" )
9. | then
10. | cam1.setLens( 5.0 );
11. | end

view line 8 to spot difference.