How to make grep match \1 \2 e.t.c in search string

Question

I tried following code to make grep search for \2 But grep fails to find the matching string in test file. I am running cygwin.

C:\test 19# val="C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise"

C:\test 20# escval="$(echo "$value" | sed -r 's,\\,\\\\,g')"

C:\test 20# echo $escval
C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Enterprise

C:\test 21# echo "export DIR=\"${val}\"" > testdata

C:\test 22# cat testdata
export DIR="C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise"

C:\test 23# cat testdata | grep -E  "^[[:space:]]*export[[:space:]]+${key}=\"${val}\""; echo $?
grep: Invalid back reference
2

C:\test 24# cat testdata |  grep -E "^[[:space:]]*export[[:space:]]+${key}=\"${escval}\""; echo $?
1

C:\test 25# grep -V
grep (GNU grep) 3.0
Packaged by Cygwin (3.0-2)
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and others, see <http://git.sv.gnu.org/cgit/grep.git/tree/AUTHORS>.

Trying with fixed strings is not an option since the actual expression..

Updates: I tried all escaping too and i have modified my question.

Answer 1

You need to escape all the backslashes, not only the one before the number (2) and with the updated question in which extended regular expressions are really needed, you need to escape ( and ) too. In fact, escape everything but alphanumeric characters to be sure:

$ escval="$(echo "$val" | sed -E 's,([^[:alnum:]]),\\\1,g')"
$ echo "${escval}"
C\:\\Program\ Files\ \(x86\)\\Microsoft\ Visual\ Studio\\2019\\Enterprise
$ grep -E "^\s*export\s+${key}=\"${escval}\"" testdata ; echo $?
export DIR="C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise"
0

Note: I use \s instead of the character class matching on [[:space:]]. It's shorter and I find it easier to read.