How to Search my vault for all notes which a) contain a property which includes “string” AND, B) whose property that includes string has a value that includes “OtherString”?

[Yurcee]

See if this matches your expectations:
[/.*?String/: /OtherString/]

Explanation:
Regular expression mode switched on with pair of / in both key and value places.

.*? in regex layman’s terms means you either have anything before String or not; good for cases where you have word compounds and you want to make a qualifier optional.

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Related:

For the value part, /OtherString/, see also this thread with my workaround:

Search function works very poorly in obsidian?

I am using the following Templater script to take over the global search used with CTRL+SHIFT+F. With this I can find all compounds with word stems properly. <%* // Get the active workspace const workspace = app.workspace; // Ensure there's an active editor const activeLeaf = workspace.activeLeaf; if (!activeLeaf || !activeLeaf.view || !activeLeaf.view.sourceMode) { new Notice("No active editor found."); return; } // Get the editor instance const editor = activeLeaf.view.sourceMode.…

In our current case, though, the closing / had to be applied in both key and value places.