It seems e.g.cat sed_data.txt  | sed 's/\b[0-9]\{3\}\b/NUMBER/g'that I must escape characters to form a regular expression. In this case I had to escape braces in order to be interpreted as a number of times.Why? I was expecting that everything would be a regex character unless escaped. I.e. the opposite.
Why do I need to escape regex characters in sed to be interpreted as regex characters?
sed;regular expression;quoting