#!/bin/bash
mytext='\\[[123 one (/)\n\\[[456 two (/)\n\\[[789 three (/)'
myvar=456
awk -v myvar=$myvar 'BEGIN{FS=OFS="\\\\n"} {s="";
for (i=1; i<=NF; i++) {
if ($i !~ myvar) {s = s (i==1 ? "" : OFS) $i;}
}
} END{print s}' <<< "$mytext"
IyEvYmluL2Jhc2gKbXl0ZXh0PSdcXFtbMTIzIG9uZSAoLylcblxcW1s0NTYgdHdvICgvKVxuXFxbWzc4OSB0aHJlZSAoLyknCm15dmFyPTQ1Ngphd2sgLXYgbXl2YXI9JG15dmFyICdCRUdJTntGUz1PRlM9IlxcXFxuIn0ge3M9IiI7Cglmb3IgKGk9MTsgaTw9TkY7IGkrKykgeyAKCQlpZiAoJGkgIX4gbXl2YXIpIHtzID0gcyAoaT09MSA/ICIiIDogT0ZTKSAkaTt9Cgl9Cn0gRU5Ee3ByaW50IHN9JyA8PDwgIiRteXRleHQiCg==