sed: -e expression #1, char NN: invalid reference \1 on `s' command's RHS

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For example, if want to change 'lNDISTINCT CHATTERING' to 'INDISTINCT CHATTERING', we may try this:

sed "s/l([A-Z])/I\1/" file.txt
which will produce the error:
"sed: -e expression #1, char 15: invalid reference \1 on `s' command's RHS"

This is because the parenthesis must be escaped thus:

sed "s/l\([A-Z]\)/I\1/" file.txt