#!/bin/bash
for s in abcdefghijklmnop _test_ 123abc A_b1C_23 _! magical\ pony1; do
echo -n "$s" | perl -pe '$_=!/\W|^\d//2**/^_/'
echo
done
IyEvYmluL2Jhc2gKCmZvciBzIGluIGFiY2RlZmdoaWprbG1ub3AgX3Rlc3RfIDEyM2FiYyBBX2IxQ18yMyBfISBtYWdpY2FsXCBwb255MTsgZG8KCWVjaG8gLW4gIiRzIiB8IHBlcmwgLXBlICckXz0hL1xXfF5cZC8vMioqL15fLycKCWVjaG8KZG9uZQ==