Check for underscore wrapped any valid Windows filename characters except underscore and csv extension

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I'm still very new to regex, so I think I can determine whether a string ends with a case-insensitive .csv:

(\\.(?i)(csv))

However, I do not know how to check that everything before that is at least one valid Windows filename character except an underscore, one underscore, and at least one valid Windows filename character except an underscore.

How can this be done?

asked 54 secs ago

Gracchus

2,099