Depending on which NiFi Expression Language (NEL) function you are using, either of the following two NEL statements should work for you:
In above you are using the replaceAll function which expects a java regular expression. Since '\" is an escape character and NiFi is a java application, the first \ escapes the second \ and the third \ escapes the forth \. So \\\\ results in a regex of \\ being applied against the value present in the "path" variable.
In above you are using the replace function which does not path the first argument to be evaluated as a java regular expression but rather takes a string literal. And since '\' is an escape character you need \\ which tells java to treat the second \ as the literal string value to search for.
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