# Brace expansion

```ruby
func brace_expand (input) {
    var current = ['']
    var stack = [[current]]

    loop {
        var t = input.match(
            /\G ((?:[^\\{,}]++ | \\(?:.|\z))++ | . )/gx
        )[0] \\ break

        if (t == '{') {
            stack << [current = ['']]
        }
        elsif ((t == ',') && (stack.len > 1)) {
            stack[-1] << (current = [''])
        }
        elsif ((t == '}') && (stack.len > 1)) {
            var group = stack.pop
            current   = stack[-1][-1]

            # handle the case of brace pairs without commas:
            group[0][] = group[0].map{ '{'+_+'}' }... if (group.len == 1)

            current[] = current.map { |c|
                group.map { .map { c + _ }... }...
            }...
        }
        else {
            current[] = current.map { _ + t }...
        }
    }

    # handle the case of missing closing braces:
    while (stack.len > 1) {

        var right = stack[-1].pop
        var sep = ','

        if (stack[-1].is_empty) {
            sep = '{'
            stack.pop
        }

        current = stack[-1][-1]
        current[] = current.map  { |c|
            right.map { c + sep + _ }...
        }...
    }

    return current
}

DATA.each { |line|
    say line
    brace_expand(line).each { "\t#{_}".say }
    say ''
}

__DATA__
~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
```

## Output:

```
~/{Downloads,Pictures}/*.{jpg,gif,png}
        ~/Downloads/*.jpg
        ~/Downloads/*.gif
        ~/Downloads/*.png
        ~/Pictures/*.jpg
        ~/Pictures/*.gif
        ~/Pictures/*.png

It{{em,alic}iz,erat}e{d,}, please.
        Itemized, please.
        Itemize, please.
        Italicized, please.
        Italicize, please.
        Iterated, please.
        Iterate, please.

{,{,gotta have{ ,\, again\, }}more }cowbell!
        cowbell!
        more cowbell!
        gotta have more cowbell!
        gotta have\, again\, more cowbell!

{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
        {}} some }{,{\\ edge \,}{ cases, {here} \\\\\}
        {}} some }{,{\\ edge \,}{ cases, {here} \\\\\}
```


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