Function within an expression

Is it possible to do a function inside of an expression?

Here is my XSL code:

<xsl:for-each select="$lines">
				
<record>
					
<xsl:apply-templates select="$field_definitions/*[@type = (substring(., 1,
3))]">
		
<xsl:sort select="field_no" order="ascending" data-type="number"/>
		
<xsl:with-param name="current_context" select="."/>
						
</xsl:apply-templates>
				
</record>

</xsl:for-each> 



My question is in regards to the substring function in the apply-templates
expression.  

The current context of the $lines variable will look something like this:

"AA0 12345 fdsajl129..."  

I'm trying to match the first three characters of that string.

If I can't do this, can I create a variable for this for-each ONLY?  

Thanks again,
Jim neff