#!/bin/bash
s='>AX-89916436-Affx-G-[A/G]
XXXXXXX
>AX-89916437-Affx-A-[A/G]
XXXXXXXXXXX
>AX-89916438-Affx-C-[A/C]
XXXXXXX
>AX-89916440-Affx-T-[G/T]
XXXXXXX'
awk 'BEGIN{FS=OFS="-"} /^>/ && $5 ~ /\[[A-Z]\/[A-Z]]/{gsub("[][/"$4"]", "", $5);}1' <<< "$s"
IyEvYmluL2Jhc2gKcz0nPkFYLTg5OTE2NDM2LUFmZngtRy1bQS9HXQpYWFhYWFhYCj5BWC04OTkxNjQzNy1BZmZ4LUEtW0EvR10KWFhYWFhYWFhYWFgKPkFYLTg5OTE2NDM4LUFmZngtQy1bQS9DXQpYWFhYWFhYCj5BWC04OTkxNjQ0MC1BZmZ4LVQtW0cvVF0KWFhYWFhYWCcKCmF3ayAnQkVHSU57RlM9T0ZTPSItIn0gL14+LyAmJiAkNSB+IC9cW1tBLVpdXC9bQS1aXV0ve2dzdWIoIltdWy8iJDQiXSIsICIiLCAkNSk7fTEnIDw8PCAiJHMi