Search: ^(\d+).*
Search Field: title
Replace: SeriesName [\1]
Replace Field: series
The ^ in the search marks the start of the line
The () in the search mark a section of the search that's to be used to replace the \1 in the replace field - in this case, \d+, one or more contiguous digits.
the .* is just to eliminate the rest of the line.
Since we're replacing the complete series field, you need to include the series name in the replace field, so use the actual series name instead of SeriesName.
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