#!/bin/bash
s="[abc]
field1=34234
field2=t2342

[def]
field1=44354
field2=23423"

awk -v RS= '/23423/{gsub(/[][]/, "", $1); print $1}' <<< "$s"
awk -v RS= '/23423/{print substr($1, 2, length($1)-2)}' <<< "$s"