[xsl] identifying series of numbers in text strings

[Joyce Celeste Chapman <chapmajc@xxxxxxxxxxxxx>]

Dear all,

Using XSL 1.0, is there a way I can search the contents of an element 
to find series of 4-digit numbers? Then if there are two sets of 
4-digit numbers, I would like to select those values in order (I'll be 
outputting them somewhere else as a date span), if there is only one 
4-digit set I'll need to output that a different way. Example of XML 
below. Thanks to all in advance for the help!

<date>23 July 2001-April 2006</date>
<date>July 1996</date>
<date>unknown</date>