based on the example mymetadata.lrdevplugin, there is a new metadata field "myString" introduced.
This field is searchable, browsable.
In the library filter (I hope this is correct, because I using another GUI localication)
This field is available and I can see the values. I can select a value --> all photos are filtered with this value.
Is there any way to split the values in this field by a separator, so that I can filter on the single values.
Photo X: myString = "A; B".
Photo Y: myString = "A; C".
In the library filter I see, splitted by a separator (e.g. ";")
If I select A, the photos X, Y are selected.
If I select C, the photo Y is selected.
Or must I use keywords for such a behavior
The builtin Library Metadata browser won't do what you want. However, you can search for individual words in the metadata field using the text criterion Any Searchabled Plug-in Field:
You can't select individual plugin metadata fields, so the search might accidentally include other plugins' fields. If you want more control over that search, limiting to a specific field and with a broader array of text operators (including pattern matching), you could use my Any Filter plugin.