I do want to replace all non-slash characters with the letter x, but only on a portion of the input string. Specifically, the part between the quotes in file="some/path". And actually, I didn't make it clear, but there will be text before and after the file="some/path", so I have to be careful about what I substitute. The actual input line would look more like:

2009-06-10 19:37:29 GMT [more text here] file="some/path" status=0 des
+tination="some more text"
[download]

And the only thing I want to filter is the path inside of the file="some/path" stuff.