The number of backslashes in the data doesn't matter, it is the number of backslashes in the regex string I was talking about. Backslashes normally need to be escaped (with a backslash), however, sometimes these backslashes have to be escaped as well, hence the need for 4 backslashes to represent a single backslash. Try something like this (which allows for escaped (backslashed) commas in all the columns). (?<Col1>.+?(?<!\\\\)),(?<Col2>.+?(?<!\\\\)),(?<Col3>.+?(?<!\\\\)),(?<Col4>.+?(?<!\\\\)),(?<Col5>.+?(?<!\\\\)),(?<Col6>.+?(?<!\\\\)),(?<Col7>.+?(?<!\\\\)),(?<Col8>.+?(?<!\\\\)),(?<Col9>.+?(?<!\\\\)),(?<Col10>.+?(?<!\\\\)),(?<Col11>.+?(?<!\\\\))$ Again, it might be that you only need two backslashes each time instead of four.
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