const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>a.replace(/(.*)\[([^\[\]]*)\](.)(.*)/,(a,b,c,d,e)=>f(b+c.repeat(d)+e))

d.innerHTML=test.map(f).join("<br>");

<p id="d">

const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>a.replace(/(.*)\[([^[]*?)](.)(.*)/,(a,b,c,d,e)=>f(b+c.repeat(d)+e))

d.innerHTML=test.map(f).join("<br>");

<p id="d">

const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>a.replace(/(.*)\[([^\[\]]*)\](.)(.*)/,(a,b,c,d,e)=>f(b+c.repeat(d)+e))

d.innerHTML=test.map(f).join("<br>");

<p id="d">

const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>a.replace(/(.*)\[([^\[\]]*)\](.)(.*)/,(a,b,c,d,e)=>f(b+c.repeat(d)+e))

d.innerHTML=test.map(f).join("<br>");

<p id="d">

const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>(m=/(.*)\[([^\[\]]*)\](.)(.*)/.exec(a))?f(m[1]+m[2].repeat(m[3])+m[4]):a

d.innerHTML=test.map(f).join("<br>");

<p id="d">

const test = ["[Foo[Bar]3]2", "[one]1[two]2[three]3", "[three[two[one]1]2]3", "[!@#[$%^[&*(]2]2]2", "[[foo bar baz]1]1", "[only once]12", "[only twice]23456789", "[remove me!]0", "before [in ]2after"];

const f=a=>a.replace(/(.*)\[([^\[\]]*)\](.)(.*)/,(a,b,c,d,e)=>f(b+c.repeat(d)+e))

d.innerHTML=test.map(f).join("<br>");

<p id="d">