Using 'contains' function with xsl:key

Hi everyone!
I have a XML with elements that have an id attribute.
The value of the id attribute is composed in one of the two ways:

1) '_field:' + valid string + number
2)'_group:' + valid string + number

All i want is to create two different groups based on the first word
of the attribute value : '_field'  or 'group'.

One way is:
<xsl:key name="ctlkey" match="*" use="substring-before(@id,':')"/>

and use it
key('ctlkey','_field') or key('ctlkey','_group')

now my quetion is:
Is it possible to have the xsl:key defined:

<xsl:key name="ctlkey" match="*" use="contains(@id,'_field')"/>

and then use it

key('ctlkey',true()) or key('ctlkey',false()) ??????

thanks anyway