This can be done in two statements. In both cases the statement is Indexed Artwork.

Get index of artwork item matching description "test" to Variable 0
Set description of artwork item[\v0] to "Hello", per file index

The first statement returns the index of the first artwork item to match some text. (In this case 'test'). The matched index is returned in track variable 0. If there is no match for a particular file, the variable is set to -1.

The second statement sets sets the description field of the artwork item at the index in per file track variable 0. Note if the index is -1 it is invalid and ignored. In this sample, the description is set to 'Hello'. Note that per file must be specified as the track variable can have a different value in each file.

Note that the search is always case insensitive and that as escape sequences can be specified in the text fields, they can be set at runtime. So the following could be done:

Prompt for Text save to named variable 'pattern', "Enter the pattern:"
Prompt for Text save to named variable 'replacement', "Replace the pattern with:"
Get index of artwork item matching description "\<pattern>" to Variable 0
Set description of artwork item[\v0] to "\<replacement>", per file index

Brilliant, thank you very much. Much appreciated!