I disagree with the expected result.

$1 refers to the string captured by (a).

In this case (a) will match 'a', (b) with match 'b' etc.
When matching (a), the captured string $1 will be 'a', ptherwise, it will be the empty string.

Therefore I expected 

       <assert-string-value>ak</assert-string-value>

to hold.


   <test-case name="fn-replace-55">
      <description>Test case with undefined captures. Saxon bug 2865.</description>
      <created by="Michael Kay after Gunther Rademacher" on="2016-07-31"/>
      <test>replace('abcdefghijk', '(a)|(b)|(c)|(d)|(e)|(f)|(g)|(h)|(i)|(j)', '$1')
      </test>
      <result>
         <assert-string-value>abcdefghijk</assert-string-value>
      </result>
   </test-case>