Why does regex with \\$\{ work with egrep, but not with sed?

Question

Given a text like this

./RFF_09 -f${FILE} -c${COND}

inside a file, this egrep command will correctly match:

egrep './RFF(.*) (.*)-c\\$\{COND\}' file

but this sed command will not

sed -n "s:'./RFF(.*) (.*)-c\\$\{COND\}':./RFF$1 $2-cRFF$1:gp"

It will fail with sed: -e expression #1, char 38: invalid content of \{\}. I've also tried with

sed -n "s:'./RFF(.*) (.*)-c\\$\{COND\}':DUMMY:gp" file
sed -n s:'./RFF(.*) (.*)-c\\$\{COND\}':DUMMY:gp file

with the same result.

sed -n "s:'./RFF(.*) (.*)-c\\$\\{COND\\}':DUMMY:gp" file

Will not give me an error message, but will not match.

What am I doing wrong? Or better: How can I replace the text as suggested by the original command? I'm using rather old versions of sed (4.1.2) and egrep (2.5.1), so a workaround is appreciated even if you can't reproduce the error with newer versions.

Answer 1

If you use egrep that mean grep with extended regexp syntax so to be able to transfer your pattern into sed you have to add parametr -r(--regexp-extended) or -E in some versions.

Regarding to your expression you have extra \ after c so even with egrep it does not match

Additionally your better use \1 instead $1 for reverse-matching.

So final command could be:

sed -rn 's:\./(RFF.*)( .* -c)\$\{COND\}:./\1\2\1:gp'

Or

sed -rn 's:(\./(RFF\S*) \S* -c)\$\{COND\}:\1\2:gp'

Answer 2

In your original statement the problem is that you're mixing syntactic and literal quotes: the single quotes within the double quotes will be matched literally. You'll want to either simply remove them or mix quotes (not nest them). Untested examples:

sed 's/foo/{bar}/'
sed "s/foo/"'{bar}'"/"

There are many flavours of regular expressions, and they all support different syntax. In the attempt using only single quotes the issue is the regex flavour: sed uses \{N\} for match counts, so N needs to be an integer. You'll want to instead use {COND}.

(Truly, using a regex to solve a problem now means you have two problems. And doing pretty much anything complex in shell code means you have N problems, N being at least as large as the number of lines.)