const regex = /([a-zA-Z_]\w+|[a-zA-Z]\w*|\[\s*[a-zA-Z_]\w+\s*\]|\[\s*[a-zA-Z]\w*\s*\])\s*\[\s*['"]([a-zA-Z_]\w+|[a-zA-Z]\w*)['"]\s*\]/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('([a-zA-Z_]\\w+|[a-zA-Z]\\w*|\\[\\s*[a-zA-Z_]\\w+\\s*\\]|\\[\\s*[a-zA-Z]\\w*\\s*\\])\\s*\\[\\s*[\'"]([a-zA-Z_]\\w+|[a-zA-Z]\\w*)[\'"]\\s*\\]', 'gm')
const str = `a['b'][c]
a['1']
a['b1']
a[1]
a['_'] --I am pretty sure this is bad to match
a["__"] --idk if this is bad to match
a["_ _"]
a["up"]
a["up "]
a['b']['c'] --notice how this doesn't match the first quote with the last one
a["workwork"]
_adf [ 'test' ] --even when imporoperlly formated this is valid lua
a.[b] ["yup"] --this case preserves the [b] and will append .yup after it
a["a.basdf"]
"adlfkajsd.dklajdlakj"`;
const subst = `$1.$2`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions