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The important settings here are the multiline.* ones which manage multiline formatted logs. The .pattern matches lines starting with white-space. The .negate and .match work together, and combined as false and after make consecutive lines that match the pattern to be appended to the previous line that doesn't match it. This makes all the lines starting with whiteswhite-pace space to be appended to the line that hold the date, actually the first line in the radius_sample_logs. For more information see Manage multiline messages.
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