If you're just setting the source to a static value, you can do it via transforms, as kristian.kolb suggested, but it might be simpler to just do:
[monitor:///my/log/path/BEK*.log]
sourcetype=BEKlogs
source=BEK.log
However, if you're going to do that, I might just suggest you ignore "source" and use "sourcetype" anyway. If on the other hand, you want to preserve part of the source path, e.g., you are monitoring files like:
[monitor:///my/path/*/logs/BEK*.log]
and you want the source to read like /my/path/group1/logs/BEK.log , you would use kristian's method of a transform, but you would need a more complex REGEX and FORMAT to extract and use the appropriate parts of the source you want.
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