Special Replacements in Sed

I'm exploring a bunch of data in Java. The debugger in IntelliJ is powerful, but I find it much easier to export the data to JSON and use Python. I used GSON to export the data, but the resulting files were full of stuff like this:

"expression": "\u0026sum\u0026 ( ~input~ ) \u003e 0"

I recognized those as unicode escape sequences but didn't know which characters they represented off the top of my head. Typing <Ctrl-V>u0026 into vim inserted an ampersand. Then I turned to sed to fix up the files:

$ sed -i 's/\u0026/&/g' data.json

Which didn't work. Turns out an ampersand means something special to sed:

The replacement may contain the special character & to refer to that portion of the pattern space which matched, and the special escapes \1 through \9 to refer to the corresponding matching sub-expressions in the regexp.

I knew \1 through \9 referred to capturing groups but the & thing surprised me.

The solution is simply to escape the ampersand. However, because \u also means something special (uppercase, I guess), it also has to be escaped:

$ sed -i 's/\\u0026/\&/g' data.json

Turns out awk treats & in replacements the same way. Go figure.

Thanks goes once again to Gilles, whose infinite wisdom has helped me on countless occasions.

The sed manual is available here or with info sed. man sed is lacking in comparison.