intake problems with _ (underscore)

[Diogo Saad]

I'm using intake in my webapp to validate some form fields.
I get errors if I use _ (underscore caracter) in intake.xml

Eg: mapToProperty="Key_Number"    DOES NOT WORK     
       mapToProperty="KeyNumber"    WORKS

<group name="Car" key="c" mapToObject="om.Car">
<field name="KeyNum" key="k" type="String" mapToProperty="Key_Number">
       <rule name="mask"  value="^([0-9]*)$">wrong syntax .</rule> 
  </field>
</group>


Is there a problem with intake and underscores or I'm not supposed to use them anyway 
??