Assuming that "year" is any string of 
space, four digits, comma
and you want to replace them by a comma
then in xslt2 you can go
<xsl:value-of select="replace(.,' [0-9][0-9][0-9][0-9],',',')"/>
in xslt1 you'd have to try a lot harder unless you have acces sto an
extension function with regular expression support, which some xslt
systems do have.

David