[xsl] Regular expressions in XSLT 2

["tom tom" <tomxsllist@xxxxxxxxxxx>]

I need to create a key that matches on any <t/> element and uses a substring 
that exists within an xml:id attribute after the first '_' and before the 
final group of numbers. For example

<t xml:id="tr_165_promo3650"/>

Would be held in the key

<xsl:key name="O" match="//t" use="{?????????????}"/>

and referenced with

key('O', '165_promo')

Can anyone advise on the best way to do this in XSLT 2?

Thanks

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