How to return index of an array in a user defined function in excel? -

i have user-defined function (not mine, in plug-in) returns array of values in column a. have following (array) formula in column b, returns cells in column contain specified string:

=index($a$1:$a$256,small(if(isnumber(search($b$1,$a$1:$a$256)),row($a$1:$a$256)),row(2:2))) 

this uses row() function row(s) of matching elements. works fine.

however, clean sheet little, , following:

=index(udf(),small(if(isnumber(search($b$1,udf())),row(udf())),row(2:2))) 

youll notice have replaced array '$a$1:$a$256' returned array udf. seems break @ row(udf()) part. there similar function, return index of array, or 'row'?

note: rather not in vba, , cannot edit udf (it not mine).

edit:

ok have bit more information. if suggested in comments, , evaluate formula, evaluation returns first element of array. brought related, second question. how 1 udf? if familiar bloomberg, know when call bdh or bdp formula (not calling array formula), fills range values. how happen?! feel if know answer question, me understand happening in particular udf.