Grep regex treated as path - regex

Question

I have a script, where I read strings from txt file, then assign it to $snmp_cred variable and then trying to strip ip address from strings, using grep, into another variable ($snmp_ip)
while read snmp_cred; do
echo appliance $ADDM_address and $snmp_cred
snmp_ip=$(echo $snmp_cred | grep "/((25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\d(?=#)/g")
echo IP for snmp community is $snmp_ip
done </tmp/input.txt
Content of input.txt file is:
a10networks/generic/1.3.6.1.4.1.22610.1.3.27_thunder_series4430s/10.72.168.33#public
a10networks/generic/1.3.6.1.4.1.22610.1.3.23_thunder_series1030s/172.17.48.24#public
a10networks/generic/1.3.6.1.4.1.22610.1.3.16_ax3200_12/10.251.1.101#public
The regex works in online regex editor, but fails into bash script. Bash output is:
++ echo $'a10networks/generic/1.3.6.1.4.1.22610.1.3.27_thunder_series4430s/10.72.168.33#public\r'
++ grep '/((25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\d(?=#)/g'
+ snmp_ip=
+ echo IP for snmp community is
IP for snmp community is
can anyone point, what an I doing wrong?

Answer 1

Since you are not getting the matched texts only, you do not really need the lookahead that the POSIX regex does not support. Also, note that \d is not supported by POSIX regex standard either. Also, grep pattern should not be placed inside regex delimiters.
If you still need to use your pattern (say, to also grab the matches), pass the -oP option use:
grep -oP "((25[0-5]|2[0-4]\d|[01]?[1-9]\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?[1-9]\d?)\d(?=#)"
And the online demo

Answer 2

In this statement:
snmp_ip=$($snmp_cred | grep "/((25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\d(?=#)/g")
you are just expanding the variable, without passing it to grep.
you need to either pass it to grep as an argument (in the form of a file redirection) or send it to greps STDIN.
this worked for me
#!/bin/bash
while read snmp_cred; do
#echo appliance $ADDM_address and $snmp_cred
snmp_ip=$(grep -E -o "((25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[1-9][0-9]?)#" <<< $snmp_cred)
echo IP for snmp community is $snmp_ip
done <input.txt
output:
IP for snmp community is 10.72.168.33#
IP for snmp community is 172.17.48.24#
IP for snmp community is