Bad regex causes code to fail

[greg00000]

When using these options:
{ compact: false, identifierNamesGenerator: 'mangled', stringArray: true, stringArrayEncoding: 'base64' }

This code:

main(); function main() { alert("Hello world!") }

Obfuscates as the following code, which fails due to the regex /=+$/:

var a = ['SGVsbG8gd29ybGQh']; (function (c, d) { var e = function (f) { while (--f) { c['push'](c['shift']()); } }; e(++d); }(a, 0x6f)); var b = function (c, d) { c = c - 0x0; var e = a[c]; if (b['yZthrT'] === undefined) { (function () { var f = function () { var g; try { g = Function('return\x20(function()\x20' + '{}.constructor(\x22return\x20this\x22)(\x20)' + ');')(); } catch (h) { g = window; } return g; }; var i = f(); var j = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='; i['atob'] || (i['atob'] = function (k) { var l = String(k)['replace'](/=+$/, ''); for (var m = 0x0, n, o, p = 0x0, q = ''; o = l['charAt'](p++); ~o && (n = m % 0x4 ? n * 0x40 + o : o, m++ % 0x4) ? q += String['fromCharCode'](0xff & n >> (-0x2 * m & 0x6)) : 0x0) { o = j['indexOf'](o); } return q; }); }()); b['aypUXW'] = function (r) { var s = atob(r); var t = []; for (var u = 0x0, v = s['length']; u < v; u++) { t += '%' + ('00' + s['charCodeAt'](u)['toString'](0x10))['slice'](-0x2); } return decodeURIComponent(t); }; b['EhRyAC'] = {}; b['yZthrT'] = !![]; } var w = b['EhRyAC'][c]; if (w === undefined) { e = b['aypUXW'](e); b['EhRyAC'][c] = e; } else { e = w; } return e; }; main(); function main() { alert(b('0x0')); }

[greg00000]

Tried to replicate this via https://obfuscator.io/ and it passes on that page.
But fails in ESTK (the interpreter used for using JS with Adobe products, and the basis of JS for extension panels in Photoshop, InDesign, Ilustrator, etc), which is based on ecmascript 3 I believe. Old, but an important platform for many developers.

This line specifically fails due to the different interpretation of the unescaped equals sign:
var l = String(k)'replace';