Brace expansion

From Rosetta Code
Task
Brace expansion
You are encouraged to solve this task according to the task description, using any language you may know.

Brace expansion is a type of parameter expansion made popular by Unix shells, where it allows users to specify multiple similar string parameters without having to type them all out. E.g. the parameter enable_{audio,video} would be interpreted as if both enable_audio and enable_video had been specified.

Task[edit]

Write a function that can perform brace expansion on any input string, according to the following specification.
Demonstrate how it would be used, and that it passes the four test cases given below.

Specification

In the input string, balanced pairs of braces containing comma-separated substrings (details below) represent alternations that specify multiple alternatives which are to appear at that position in the output. In general, one can imagine the information conveyed by the input string as a tree of nested alternations interspersed with literal substrings, as shown in the middle part of the following diagram:

It{{em,alic}iz,erat}e{d,}
parse 
―――――▶
It




em


alic
iz


erat
e


d


expand 
―――――▶
Itemized
Itemize
Italicized
Italicize
Iterated
Iterate
input string alternation tree output (list of strings)

This tree can in turn be transformed into the intended list of output strings by, colloquially speaking, determining all the possible ways to walk through it from left to right while only descending into one branch of each alternation one comes across (see the right part of the diagram). When implementing it, one can of course combine the parsing and expansion into a single algorithm, but this specification discusses them separately for the sake of clarity.

Expansion of alternations can be more rigorously described by these rules:

a


2


1
b


X


Y
X
c
a2bXc
a2bYc
a2bXc
a1bXc
a1bYc
a1bXc
  • An alternation causes the list of alternatives that will be produced by its parent branch to be increased 𝑛-fold, each copy featuring one of the 𝑛 alternatives produced by the alternation's child branches, in turn, at that position.
  • This means that multiple alternations inside the same branch are cumulative  (i.e. the complete list of alternatives produced by a branch is the string-concatenating "Cartesian product" of its parts).
  • All alternatives (even duplicate and empty ones) are preserved, and they are ordered like the examples demonstrate  (i.e. "lexicographically" with regard to the alternations).
  • The alternatives produced by the root branch constitute the final output.

Parsing the input string involves some additional complexity to deal with escaped characters and "incomplete" brace pairs:

a\\{\\\{b,c\,d}
a\\


\\\{b


c\,d
{a,b{c{,{d}}e}f
{a,b{c




{d}
e}f
  • An unescaped backslash which precedes another character, escapes that character (to force it to be treated as literal). The backslashes are passed along to the output unchanged.
  • Balanced brace pairs are identified by, conceptually, going through the string from left to right and associating each unescaped closing brace that is encountered with the nearest still unassociated unescaped opening brace to its left (if any). Furthermore, each unescaped comma is associated with the innermost brace pair that contains it (if any). With that in mind:
    • Each brace pair that has at least one comma associated with it, forms an alternation (whose branches are the brace pair's contents split at its commas). The associated brace and comma characters themselves do not become part of the output.
    • Brace characters from pairs without any associated comma, as well as unassociated brace and comma characters, as well as all characters that are not covered by the preceding rules, are instead treated as literals.

For every possible input string, your implementation should produce exactly the output which this specification mandates. Please comply with this even when it's inconvenient, to ensure that all implementations are comparable. However, none of the above should be interpreted as instructions (or even recommendations) for how to implement it. Try to come up with a solution that is idiomatic in your programming language. (See #Perl for a reference implementation.)

Test Cases
Input
(single string)
Ouput
(list/array of strings)

~/{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} \\\\\}


AutoHotkey[edit]

This example is incorrect. Please fix the code and remove this message.
Details: Output for edge cases is wrong.
; AutoHotkey runs Perl-compatible regular expressions no problem
t=~/{Downloads,Pictures}/ *.{jpg,gif,png} ; Note I added a space so the RosettaCode syntax highlighting will work. The AutoHotkey interpreter handles it no problem.
msgbox % expand(t)
t=It{{em,alic}iz,erat}e{d,}, please.
msgbox % expand(t)
t={,{,gotta have{ ,\, again\, }}more }cowbell!
msgbox % expand(t)
t={}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
msgbox % expand(t)
 
expand(t) {
t:=RegExReplace(t, "\\\\", "\!") ; if you want to use these character combinations literally in your string, just switch these susbstitutions to something unicode
,t:=RegExReplace(t, "\\,", "\#")
,t:=RegExReplace(t, "\\\{", "\@")
,t:=RegExReplace(t, "\\\}", "\&")
a=1
While a or b
; expand (braces with multiple commas) which are (apparently inside something else)
t:=RegExReplace(t, "Sxm)^(.*) ([{,]) ([^{},]*) \{ ([^{},]*) , ([^{},]* , [^{}]*) \} ([^{},]*?) ([},]) (.*)$", "$1$2$3$4$6,$3{$5}$6$7$8", a)
; expand (braces with single commas) which are (apparently inside something else)
,t:=RegExReplace(t, "Sxm)^(.*) ([{,]) ([^{},]*) \{ ([^{},]*) , ([^{},]*) \} ([^{},]*?) ([},]) (.*)$", "$1$2$3$4$6,$3$5$6$7$8", b)
a=1
While a or b
; expand braces with single commas
t:=RegExReplace(t, "Sxm)^(.*) \{ ([^{},]*) , ([^{},]*) \} (.*)$", "$1$2$4`r`n$1$3$4", a)
; expand braces with multiple commas
,t:=RegExReplace(t, "Sxm)^(.*) \{ ([^{},]*) , ([^{},]* , [^{}]*) \} (.*)$", "$1$2$4`r`n$1{$3}$4", b)
t:=RegExReplace(t, "\\!", "\\")
,t:=RegExReplace(t, "\\#", "\,")
,t:=RegExReplace(t, "\\@", "\{")
,t:=RegExReplace(t, "\\&", "\}")
Return t
}
Output:
~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png

Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.

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

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

C++[edit]

C++11 solution:

#include <iostream>
#include <iterator>
#include <string>
#include <utility>
#include <vector>
 
namespace detail {
 
template <typename ForwardIterator>
class tokenizer
{
 
ForwardIterator _tbegin, _tend, _end;
 
public:
 
tokenizer(ForwardIterator begin, ForwardIterator end)
: _tbegin(begin), _tend(begin), _end(end)
{ }
 
template <typename Lambda>
bool next(Lambda istoken)
{
if (_tbegin == _end) {
return false;
}
_tbegin = _tend;
for (; _tend != _end && !istoken(*_tend); ++_tend) {
if (*_tend == '\\' && std::next(_tend) != _end) {
++_tend;
}
}
if (_tend == _tbegin) {
_tend++;
}
return _tbegin != _end;
}
 
ForwardIterator begin() const { return _tbegin; }
ForwardIterator end() const { return _tend; }
bool operator==(char c) { return *_tbegin == c; }
 
};
 
template <typename List>
void append_all(List & lista, const List & listb)
{
if (listb.size() == 1) {
for (auto & a : lista) {
a += listb.back();
}
} else {
List tmp;
for (auto & a : lista) {
for (auto & b : listb) {
tmp.push_back(a + b);
}
}
lista = std::move(tmp);
}
}
 
template <typename String, typename List, typename Tokenizer>
List expand(Tokenizer & token)
{
 
std::vector<List> alts{ { String() } };
 
while (token.next([](char c) { return c == '{' || c == ',' || c == '}'; })) {
 
if (token == '{') {
append_all(alts.back(), expand<String, List>(token));
} else if (token == ',') {
alts.push_back({ String() });
} else if (token == '}') {
if (alts.size() == 1) {
for (auto & a : alts.back()) {
a = '{' + a + '}';
}
return alts.back();
} else {
for (std::size_t i = 1; i < alts.size(); i++) {
alts.front().insert(alts.front().end(),
std::make_move_iterator(std::begin(alts[i])),
std::make_move_iterator(std::end(alts[i])));
}
return std::move(alts.front());
}
} else {
for (auto & a : alts.back()) {
a.append(token.begin(), token.end());
}
}
 
}
 
List result{ String{ '{' } };
append_all(result, alts.front());
for (std::size_t i = 1; i < alts.size(); i++) {
for (auto & a : result) {
a += ',';
}
append_all(result, alts[i]);
}
return result;
}
 
} // namespace detail
 
template <
typename ForwardIterator,
typename String = std::basic_string<
typename std::iterator_traits<ForwardIterator>::value_type
>,
typename List = std::vector<String>
>
List expand(ForwardIterator begin, ForwardIterator end)
{
detail::tokenizer<ForwardIterator> token(begin, end);
List list{ String() };
while (token.next([](char c) { return c == '{'; })) {
if (token == '{') {
detail::append_all(list, detail::expand<String, List>(token));
} else {
for (auto & a : list) {
a.append(token.begin(), token.end());
}
}
}
return list;
}
 
template <
typename Range,
typename String = std::basic_string<typename Range::value_type>,
typename List = std::vector<String>
>
List expand(const Range & range)
{
using Iterator = typename Range::const_iterator;
return expand<Iterator, String, List>(std::begin(range), std::end(range));
}
 
int main()
{
 
for (std::string string : {
R"(~/{Downloads,Pictures}/*.{jpg,gif,png})",
R"(It{{em,alic}iz,erat}e{d,}, please.)",
R"({,{,gotta have{ ,\, again\, }}more }cowbell!)",
R"({}} some {\\{edge,edgy} }{ cases, here\\\})",
R"(a{b{1,2}c)",
R"(a{1,2}b}c)",
R"(a{1,{2},3}b)",
R"(a{b{1,2}c{}})",
R"(more{ darn{ cowbell,},})",
R"(ab{c,d\,e{f,g\h},i\,j{k,l\,m}n,o\,p}qr)",
R"({a,{\,b}c)",
R"(a{b,{{c}})",
R"({a{\}b,c}d)",
R"({a,b{{1,2}e}f)",
R"({}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\})",
R"({{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{)",
}) {
std::cout << string << '\n';
for (auto expansion : expand(string)) {
std::cout << " " << expansion << '\n';
}
std::cout << '\n';
}
 
return 0;
}

Common Lisp[edit]

(defstruct alternation
(alternatives nil :type list))
 
(defun alternatives-end-positions (string start)
(assert (char= (char string start) #\{))
(loop with level = 0
with end-positions
with escapep and commap
for index from start below (length string)
for c = (char string index)
do (cond (escapep
(setf escapep nil))
((char= c #\\)
(setf escapep t))
((char= c #\{)
(incf level))
((char= c #\})
(decf level)
(when (zerop level)
(push index end-positions)
(loop-finish)))
((and (char= c #\,) (= level 1))
(setf commap t)
(push index end-positions)))
finally (return (and (zerop level) commap (nreverse end-positions)))))
 
(defun parse-alternation (string start)
(loop with end-positions = (alternatives-end-positions string start)
for %start = (1+ start) then (1+ %end)
for %end in end-positions
collect (parse string :start %start :end %end) into alternatives
finally (return (and alternatives
(values (make-alternation :alternatives alternatives) (1+ %end))))))
 
(defun parse (string &key (start 0) (end (length string)))
(loop with result and escapep
for index = start then next
while (< index end)
for c = (char string index)
for next = (1+ index)
do (cond (escapep
(push c result)
(setf escapep nil))
((char= c #\\)
(push c result)
(setf escapep t))
((char= c #\{)
(multiple-value-bind (alternation next-index)
(parse-alternation string index)
(cond (alternation
(push alternation result)
(setf next next-index))
(t
(push c result)))))
(t
(push c result)))
finally (return (nreverse result))))
 
(defun traverse-alternation (alternation)
(mapcan #'traverse (alternation-alternatives alternation)))
 
(defun traverse (parsed)
(let ((results (list nil)))
(dolist (element parsed results)
(etypecase element
(character
(setf results (loop for r in results
collect (nconc r (list element)))))
(alternation
(setf results (loop for r in results
nconc (loop for ar in (traverse-alternation element)
collect (append r ar)))))))))
 
(defun expand (string)
(loop for result in (traverse (parse string))
collect (coerce result 'string)))
 
(defun main ()
(dolist (input '("~/{Downloads,Pictures}/*.{jpg,gif,png}"
"It{{em,alic}iz,erat}e{d,}, please."
"{,{,gotta have{ ,\\, again\\, }}more }cowbell!"
"{}} some }{,{\\\\{ edge, edge} \\,}{ cases, {here} \\\\\\\\\\}"))
(write-line input)
(dolist (output (expand input))
(format t " ~A~%" output))
(terpri)))
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} \\\\\}

D[edit]

Translation of: Python

This code is not UTF-corrected, because it uses slicing instead of front, popFront, etc.

import std.stdio, std.typecons, std.array, std.range, std.algorithm, std.string;
 
Nullable!(Tuple!(string[], string)) getGroup(string s, in uint depth)
pure nothrow @safe {
string[] sout;
auto comma = false;
 
while (!s.empty) {
// {const g, s} = getItems(s, depth);
const r = getItems(s, depth);
const g = r[0];
s = r[1];
 
if (s.empty)
break;
sout ~= g;
 
if (s[0] == '}') {
if (comma)
return typeof(return)(tuple(sout, s[1 .. $]));
return typeof(return)(tuple(
sout.map!(a => '{' ~ a ~ '}').array, s[1 .. $]));
}
 
if (s[0] == ',') {
comma = true;
s = s[1 .. $];
}
}
return typeof(return)();
}
 
Tuple!(string[], string) getItems(string s, in uint depth=0) pure @safe nothrow {
auto sout = [""];
 
while (!s.empty) {
auto c = s[0 .. 1];
if (depth && (c == "," || c == "}"))
return tuple(sout, s);
 
if (c == "{") {
/*const*/ auto x = getGroup(s.dropOne, depth + 1);
if (!x.isNull) {
sout = cartesianProduct(sout, x[0])
.map!(ab => ab[0] ~ ab[1])
.array;
s = x[1];
continue;
}
}
 
if (c == "\\" && s.length > 1) {
c ~= s[1];
s = s[1 .. $];
}
 
sout = sout.map!(a => a ~ c).array;
s = s[1 .. $];
}
return tuple(sout, s);
}
 
void main() @safe {
immutable testCases = r"~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
a{b{1,2}c
a{1,2}b}c
a{1,{2},3}b
a{b{1,2}c{}}
more{ darn{ cowbell,},}
ab{c,d\,e{f,g\h},i\,j{k,l\,m}n,o\,p}qr
{a,{\,b}c
a{b,{{c}}
{a{\}b,c}d
{a,b{{1,2}e}f"
;
 
foreach (const s; testCases.splitLines)
writefln("%s\n%-(  %s\n%)\n", s, s.getItems[0]);
}
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} \\\\\}

a{b{1,2}c
    a{b1c
    a{b2c

a{1,2}b}c
    a1b}c
    a2b}c

a{1,{2},3}b
    a1b
    a{2}b
    a3b

a{b{1,2}c{}}
    a{b1c{}}
    a{b2c{}}

more{ darn{ cowbell,},}
    more darn cowbell
    more darn
    more

ab{c,d\,e{f,g\h},i\,j{k,l\,m}n,o\,p}qr
    abcqr
    abd\,efqr
    abd\,eg\hqr
    abi\,jknqr
    abi\,jl\,mnqr
    abo\,pqr

{a,{\,b}c
    {a,{\,b}c

a{b,{{c}}
    a{b,{{c}}

{a{\}b,c}d
    {a\}bd
    {acd

{a,b{{1,2}e}f
    {a,b{1e}f
    {a,b{2e}f

Elixir[edit]

Translation of: Ruby
defmodule Brace_expansion do
def getitem(s), do: getitem(String.codepoints(s), 0, [""])
 
defp getitem([], _, out), do: {out,[]}
defp getitem([c|_]=s, depth, out) when depth>0 and (c == "," or c == "}"), do: {out,s}
defp getitem([c|t], depth, out) do
x = getgroup(t, depth+1, [], false)
if (c == "{") and x do
{y, z} = x
out2 = for a <- out, b <- y, do: a<>b
getitem(z, depth, out2)
else
if c == "\\" and length(t) > 0 do
c2 = c <> hd(t)
getitem(tl(t), depth, Enum.map(out, fn a -> a <> c2 end))
else
getitem(t, depth, Enum.map(out, fn a -> a <> c end))
end
end
end
 
defp getgroup([], _, _, _), do: nil
defp getgroup(s, depth, out, comma) do
{g, s2} = getitem(s, depth, [""])
if s2 == [] do
nil
else
out2 = out ++ g
case hd(s2) do
"}" -> if comma, do: {out2, tl(s2)},
else: {Enum.map(out2, &"{#{&1}}"), tl(s2)}
"," -> getgroup(tl(s2), depth, out2, true)
_ -> getgroup(s2, depth, out2, comma)
end
end
end
end
 
test_cases = ~S"""
~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
""" |> String.split("\n", trim: true)
 
Enum.each(test_cases, fn s ->
IO.puts s
Brace_expansion.getitem(s)
|> elem(0)
|> Enum.each(fn str -> IO.puts "\t#{str}" end)
IO.puts ""
end)
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} \\\\\}

Haskell[edit]

"Here is a direct translation to Haskell using parsec" (of an earlier version of the Perl 6 solution):

import Control.Applicative (liftA2, pure, (<$>), (<*>))
import Control.Monad (forever)
import Text.Parsec
 
parser :: Parsec String u [String]
parser = expand <$> many (try alts <|> try alt1 <|> escape <|> pure . pure <$> anyChar)
where
alts = concat <$> between (char '{') (char '}') (alt `sepBy2` char ',')
alt1 = (:[]).('{':).(++"}") <$> between (char '{') (char '}') (many $ noneOf ",{}")
alt = expand <$> many (try alts <|> try alt1 <|> escape <|> pure . pure <$> noneOf ",}")
escape = pure <$> sequence [char '\\', anyChar]
expand = foldr (liftA2 (++)) [""]
p `sepBy2` sep = liftA2 (:) p (many1 (sep >> p))
 
main :: IO ()
main = forever $ parse parser [] <$> getLine >>= either print (mapM_ putStrLn)
Output:
$ ./bracex
~/{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,edgy} }{ cases, here\\\}
{}} some {\\edge }{ cases, here\\\}
{}} some {\\edgy }{ cases, here\\\}
a{b{1,2}c
a{b1c
a{b2c
a{1,2}b}c
a1b}c
a2b}c
a{1,{2},3}b
a1b
a{2}b
a3b
a{b{1,2}c{}}
a{b1c{}}
a{b2c{}}
^D
bracex: <stdin>: hGetLine: end of file

Go[edit]

expand.go:

package expand
 
// Expander is anything that can be expanded into a slice of strings.
type Expander interface {
Expand() []string
}
 
// Text is a trivial Expander that expands to a slice with just itself.
type Text string
 
func (t Text) Expand() []string { return []string{string(t)} }
 
// Alternation is an Expander that expands to the union of its
// members' expansions.
type Alternation []Expander
 
func (alt Alternation) Expand() []string {
var out []string
for _, e := range alt {
out = append(out, e.Expand()...)
}
return out
}
 
// Sequence is an Expander that expands to the combined sequence of its
// members' expansions.
type Sequence []Expander
 
func (seq Sequence) Expand() []string {
if len(seq) == 0 {
return nil
}
out := seq[0].Expand()
for _, e := range seq[1:] {
out = combine(out, e.Expand())
}
return out
}
 
func combine(al, bl []string) []string {
out := make([]string, 0, len(al)*len(bl))
for _, a := range al {
for _, b := range bl {
out = append(out, a+b)
}
}
return out
}
 
// Currently only single byte runes are supported for these.
const (
escape = '\\'
altStart = '{'
altEnd = '}'
altSep = ','
)
 
type piT struct{ pos, cnt, depth int }
 
type Brace string
 
// Expand takes an input string and returns the expanded list of
// strings. The input string can contain any number of nested
// alternation specifications of the form "{A,B}" which is expanded to
// the strings "A", then "B".
//
// E.g. Expand("a{2,1}b{X,Y,X}c") returns ["a2bXc", "a2bYc", "a2bXc",
// "a1bXc", "a1bYc", "a1bXc"].
//
// Unmatched '{', ',', and '}' characters are passed through to the
// output. The special meaning of these characters can be escaped with
// '\', (which itself can be escaped).
// Escape characters are not removed, but passed through to the output.
func Expand(s string) []string { return Brace(s).Expand() }
func (b Brace) Expand() []string { return b.Expander().Expand() }
func (b Brace) Expander() Expander {
s := string(b)
//log.Printf("Exapand(%#q)\n", s)
var posInfo []piT
var stack []int // indexes into posInfo
removePosInfo := func(i int) {
end := len(posInfo) - 1
copy(posInfo[i:end], posInfo[i+1:])
posInfo = posInfo[:end]
}
 
inEscape := false
for i, r := range s {
if inEscape {
inEscape = false
continue
}
switch r {
case escape:
inEscape = true
case altStart:
stack = append(stack, len(posInfo))
posInfo = append(posInfo, piT{i, 0, len(stack)})
case altEnd:
if len(stack) == 0 {
continue
}
si := len(stack) - 1
pi := stack[si]
if posInfo[pi].cnt == 0 {
removePosInfo(pi)
for pi < len(posInfo) {
if posInfo[pi].depth == len(stack) {
removePosInfo(pi)
} else {
pi++
}
}
} else {
posInfo = append(posInfo, piT{i, -2, len(stack)})
}
stack = stack[:si]
case altSep:
if len(stack) == 0 {
continue
}
posInfo = append(posInfo, piT{i, -1, len(stack)})
posInfo[stack[len(stack)-1]].cnt++
}
}
//log.Println("stack:", stack)
for len(stack) > 0 {
si := len(stack) - 1
pi := stack[si]
depth := posInfo[pi].depth
removePosInfo(pi)
for pi < len(posInfo) {
if posInfo[pi].depth == depth {
removePosInfo(pi)
} else {
pi++
}
}
stack = stack[:si]
}
return buildExp(s, 0, posInfo)
}
 
func buildExp(s string, off int, info []piT) Expander {
if len(info) == 0 {
return Text(s)
}
//log.Printf("buildExp(%#q, %d, %v)\n", s, off, info)
var seq Sequence
i := 0
var dj, j, depth int
for dk, piK := range info {
k := piK.pos - off
switch s[k] {
case altStart:
if depth == 0 {
dj = dk
j = k
depth = piK.depth
}
case altEnd:
if piK.depth != depth {
continue
}
if j > i {
seq = append(seq, Text(s[i:j]))
}
alt := buildAlt(s[j+1:k], depth, j+1+off, info[dj+1:dk])
seq = append(seq, alt)
i = k + 1
depth = 0
}
}
if j := len(s); j > i {
seq = append(seq, Text(s[i:j]))
}
if len(seq) == 1 {
return seq[0]
}
return seq
}
 
func buildAlt(s string, depth, off int, info []piT) Alternation {
//log.Printf("buildAlt(%#q, %d, %d, %v)\n", s, depth, off, info)
var alt Alternation
i := 0
var di int
for dk, piK := range info {
if piK.depth != depth {
continue
}
if k := piK.pos - off; s[k] == altSep {
sub := buildExp(s[i:k], i+off, info[di:dk])
alt = append(alt, sub)
i = k + 1
di = dk + 1
}
}
sub := buildExp(s[i:], i+off, info[di:])
alt = append(alt, sub)
return alt
}

expand_test.go

package expand
 
import (
"fmt"
"reflect"
"testing"
)
 
// These could be in expand.go but for now they're only used for debugging.
func (t Text) String() string { return fmt.Sprintf("%#q", string(t)) }
func (alt Alternation) String() string { return fmt.Sprintf("ALT:%v", []Expander(alt)) }
 
// From http://rosettacode.org/wiki/Brace_expansion
var testCases = []struct {
input string
expected []string
}{
{"", []string{""}},
{"a{2,1}b{X,Y,X}c", []string{
"a2bXc",
"a2bYc",
"a2bXc",
"a1bXc",
"a1bYc",
"a1bXc"}},
{`a\\{\\\{b,c\,d}`, []string{
`a\\\\\{b`,
`a\\c\,d`}},
{"{a,b{c{,{d}}e}f", []string{
"{a,b{ce}f",
"{a,b{c{d}e}f"}},
{"~/{Downloads,Pictures}/*.{jpg,gif,png}", []string{
"~/Downloads/*.jpg",
"~/Downloads/*.gif",
"~/Downloads/*.png",
"~/Pictures/*.jpg",
"~/Pictures/*.gif",
"~/Pictures/*.png"}},
{"It{{em,alic}iz,erat}e{d,}, please.", []string{
"Itemized, please.",
"Itemize, please.",
"Italicized, please.",
"Italicize, please.",
"Iterated, please.",
"Iterate, please."}},
{`{,{,gotta have{ ,\, again\, }}more }cowbell!`, []string{
"cowbell!",
"more cowbell!",
"gotta have more cowbell!",
`gotta have\, again\, more cowbell!`}},
{`{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}`, []string{
`{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}`,
`{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}`}},
}
 
//func ml(l []string) string { return "\n\t" + strings.Join(l, "\n\t") }
func ml(l []string) string {
var result string
for _, s := range l {
result += fmt.Sprintf("\n\t%#q", s)
}
return result
}
 
func TestExpand(t *testing.T) {
for _, d := range testCases {
if g := Expand(d.input); !reflect.DeepEqual(g, d.expected) {
t.Errorf("unexpected result\n Expand(%#q) gave:%v\nExpected:%v",
d.input, ml(g), ml(d.expected))
} else {
// Normally Go tests aren't this verbose, but for rosettacode
t.Logf("as expected\n Expand(%#q):%v", d.input, ml(g))
}
}
}
 
func BenchmarkExpand(b *testing.B) {
input := testCases[5].input
//b.Logf("Benchmarking Expand(%#q)\n", input)
b.ReportAllocs()
for i := 0; i < b.N; i++ {
Expand(input)
}
}
Output:
% go test -v -bench=.
=== RUN TestExpand
--- PASS: TestExpand (0.00 seconds)
	expand_test.go:72: as expected
		 Expand(``):
			``
	expand_test.go:72: as expected
		 Expand(`a{2,1}b{X,Y,X}c`):
			`a2bXc`
			`a2bYc`
			`a2bXc`
			`a1bXc`
			`a1bYc`
			`a1bXc`
	expand_test.go:72: as expected
		 Expand(`a\\{\\\{b,c\,d}`):
			`a\\\\\{b`
			`a\\c\,d`
	expand_test.go:72: as expected
		 Expand(`{a,b{c{,{d}}e}f`):
			`{a,b{ce}f`
			`{a,b{c{d}e}f`
	expand_test.go:72: as expected
		 Expand(`~/{Downloads,Pictures}/*.{jpg,gif,png}`):
			`~/Downloads/*.jpg`
			`~/Downloads/*.gif`
			`~/Downloads/*.png`
			`~/Pictures/*.jpg`
			`~/Pictures/*.gif`
			`~/Pictures/*.png`
	expand_test.go:72: as expected
		 Expand(`It{{em,alic}iz,erat}e{d,}, please.`):
			`Itemized, please.`
			`Itemize, please.`
			`Italicized, please.`
			`Italicize, please.`
			`Iterated, please.`
			`Iterate, please.`
	expand_test.go:72: as expected
		 Expand(`{,{,gotta have{ ,\, again\, }}more }cowbell!`):
			`cowbell!`
			`more cowbell!`
			`gotta have more cowbell!`
			`gotta have\, again\, more cowbell!`
	expand_test.go:72: as expected
		 Expand(`{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}`):
			`{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}`
			`{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}`
PASS
BenchmarkExpand	  500000	      6562 ns/op	    2467 B/op	      64 allocs/op
ok  	rosetta_code/Brace_expansion	3.347s

J[edit]

Implementation:

 
NB. legit { , and } do not follow a legit backslash:
legit=: 1,_1}.4>(3;(_2[\"1".;._2]0 :0);('\';a.);0 _1 0 1)&;:&.(' '&,)
2 1 1 1 NB. result 0 or 1: initial state
2 2 1 2 NB. result 2 or 3: after receiving a non backslash
1 2 1 2 NB. result 4 or 5: after receiving a backslash
)
 
expand=:3 :0
Ch=. u:inv y
M=. N=. 1+>./ Ch
Ch=. Ch*-_1^legit y
delim=. 'left comma right'=. u:inv '{,}'
J=. i.K=. #Ch
while. M=. M+1 do.
candidates=. i.0 2
for_check.I. comma=Ch do.
begin=. >./I. left=check{. Ch
end=. check+<./I. right=check}. Ch
if. K>:end-begin do.
candidates=. candidates,begin,end
end.
end.
if. 0=#candidates do. break. end.
'begin end'=. candidates{~(i.>./) -/"1 candidates
ndx=. I.(begin<:J)*(end>:J)*Ch e. delim
Ch=. M ndx} Ch
end.
T=. ,<Ch
for_mark. |.N}.i.M do.
T=. ; mark divide each T
end.
u: each |each T
)
 
divide=:4 :0
if. -.x e. y do. ,<y return. end.
mask=. x=y
prefix=. < y #~ -.+./\ mask
suffix=. < y #~ -.+./\. mask
options=. }:mask <;._1 y
prefix,each options,each suffix
)

Examples:

   >expand t1
~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png
> expand t2
Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.
>expand t3
cowbell!
more cowbell!
gotta have more cowbell!
gotta have\, again\, more cowbell!
>expand t4
{}} some {\\edge }{ cases, here\\\}
{}} some {\\edgy }{ cases, here\\\}

Explanation:

Instead of working directly with text, work with a string of numeric unicode values. Negate the numbers for characters which are "off limits" because of preceding backslashes (we will take the absolute value and convert back to unicode for the final result). Also, find a limit value larger than that of the largest in-use character.

Then, iteratively: for each relevant comma, find the location of the closest surrounding braces. From these candidates, pick a pair of braces that's the shortest distance apart. Mark those braces and their contained relevant commas by replacing their character codes with an integer larger than any previously used (all of them in the set get marked with the same number). Repeat until we cannot find any more possibilities.

Finally, for each integer that we've used to mark delimiter locations, split out each of the marked options (each with a copy of that group's prefix and suffix). (Then when all that is done, take the absolute values convert back to unicode for the final result.)

Java[edit]

Should be able to handle all printable Unicode.

public class BraceExpansion {
 
public static void main(String[] args) {
for (String s : new String[]{"It{{em,alic}iz,erat}e{d,}, please.",
"~/{Downloads,Pictures}/*.{jpg,gif,png}",
"{,{,gotta have{ ,\\, again\\, }}more }cowbell!",
"{}} some }{,{\\\\{ edge, edge} \\,}{ cases, {here} \\\\\\\\\\}"}) {
System.out.println();
expand(s);
}
}
 
public static void expand(String s) {
expandR("", s, "");
}
 
private static void expandR(String pre, String s, String suf) {
int i1 = -1, i2 = 0;
String noEscape = s.replaceAll("([\\\\]{2}|[\\\\][,}{])", " ");
StringBuilder sb = null;
 
outer:
while ((i1 = noEscape.indexOf('{', i1 + 1)) != -1) {
i2 = i1 + 1;
sb = new StringBuilder(s);
for (int depth = 1; i2 < s.length() && depth > 0; i2++) {
char c = noEscape.charAt(i2);
depth = (c == '{') ? ++depth : depth;
depth = (c == '}') ? --depth : depth;
if (c == ',' && depth == 1) {
sb.setCharAt(i2, '\u0000');
} else if (c == '}' && depth == 0 && sb.indexOf("\u0000") != -1)
break outer;
}
}
if (i1 == -1) {
if (suf.length() > 0)
expandR(pre + s, suf, "");
else
System.out.printf("%s%s%s%n", pre, s, suf);
} else {
for (String m : sb.substring(i1 + 1, i2).split("\u0000", -1))
expandR(pre + s.substring(0, i1), m, s.substring(i2 + 1) + suf);
}
}
}
Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.

~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png

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

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


JavaScript[edit]

ES5 Functional[edit]

Without importing Node.js libraries, JavaScript doesn't immediately have access to anything like Haskell's Parsec, but using a functional idiom of JavaScript, and emphasising clarity more than optimisation, we can separate out the tokenizing from the parsing, and the parsing from the generation of strings, to build a function which:

  1. returns the set of expansions for each brace expression, and
  2. logs a pretty-printed abstract syntax tree for each expression to the console (in a JSON format).

Each node of the parse tree consists of one of two simple functions (AND: syntagmatic concatenation, OR: flattening of paradigms) with a set of arguments, each of which may be a plain string or an AND or OR subtree. The expansions are derived by evaluating the parse tree as an expression.

(function () {
'use strict'
 
// Index of any closing brace matching the opening brace at iPosn
// with the indices of any immediately-enclosed commas
function bracePair(tkns, iPosn, iNest, lstCommas) {
if (iPosn >= tkns.length || iPosn < 0) return null;
 
var t = tkns[iPosn],
n = (t === '{') ? iNest + 1 : (t === '}' ? iNest - 1 : iNest),
lst = (t === ',' && iNest === 1) ? lstCommas.concat(iPosn) : lstCommas;
 
return n ? bracePair(tkns, iPosn + 1, n, lst) : {
close: iPosn,
commas: lst
};
}
 
// Parse of a SYNTAGM subtree
function andTree(dctSofar, tkns) {
if (!tkns.length) return [dctSofar, []];
 
var dctParse = dctSofar ? dctSofar : {
fn: and,
args: []
},
 
head = tkns[0],
tail = head ? tkns.slice(1) : [],
 
dctBrace = head === '{' ? bracePair(
tkns, 0, 0, []
) : null,
 
lstOR = dctBrace && dctBrace.close && dctBrace.commas.length ? (
splitAt(dctBrace.close + 1, tkns)
) : null;
 
return andTree({
fn: and,
args: dctParse.args.concat(
lstOR ? orTree(dctParse, lstOR[0], dctBrace.commas) : head
)
}, lstOR ? lstOR[1] : tail);
}
 
// Parse of a PARADIGM subtree
function orTree(dctSofar, tkns, lstCommas) {
if (!tkns.length) return [dctSofar, []];
var iLast = lstCommas.length;
 
return {
fn: or,
args: splitsAt(
lstCommas, tkns
).map(function (x, i) {
var ts = x.slice(1, i === iLast ? -1 : void 0);
 
return ts.length ? ts : [''];
}).map(function (ts) {
return ts.length > 1 ? andTree(null, ts)[0] : ts[0];
})
};
}
 
// List of unescaped braces and commas, and remaining strings
function tokens(str) {
// Filter function excludes empty splitting artefacts
var toS = function (x) {
return x.toString();
};
 
return str.split(/(\\\\)/).filter(toS).reduce(function (a, s) {
return a.concat(s.charAt(0) === '\\' ? s : s.split(
/(\\*[{,}])/
).filter(toS));
}, []);
}
 
// PARSE TREE OPERATOR (1 of 2)
// Each possible head * each possible tail
function and(args) {
var lng = args.length,
head = lng ? args[0] : null,
lstHead = "string" === typeof head ? [head] : head;
 
return lng ? (
1 < lng ? lstHead.reduce(function (a, h) {
return a.concat(and(args.slice(1)).map(function (t) {
return h + t;
}));
}, []) : lstHead
) : [];
}
 
// PARSE TREE OPERATOR (2 of 2)
// Each option flattened
function or(args) {
return args.reduce(function (a, b) {
return a.concat(b);
}, []);
}
 
// One list split into two (first sublist length n)
function splitAt(n, lst) {
return n < lst.length + 1 ? [lst.slice(0, n), lst.slice(n)] : [lst, []];
}
 
// One list split into several (sublist lengths [n])
function splitsAt(lstN, lst) {
return lstN.reduceRight(function (a, x) {
return splitAt(x, a[0]).concat(a.slice(1));
}, [lst]);
}
 
// Value of the parse tree
function evaluated(e) {
return typeof e === 'string' ? e :
e.fn(e.args.map(evaluated));
}
 
// JSON prettyprint (for parse tree, token list etc)
function pp(e) {
return JSON.stringify(e, function (k, v) {
return typeof v === 'function' ? (
'[function ' + v.name + ']'
) : v;
}, 2)
}
 
 
// MAIN
 
// s -> [s]
function expansions(s) {
// BRACE EXPRESSION PARSED
var dctParse = andTree(null, tokens(s))[0];
 
// ABSTRACT SYNTAX TREE LOGGED
console.log(pp(dctParse));
 
// AST EVALUATED TO LIST OF STRINGS
return evaluated(dctParse);
}
 
 
// Sample expressions, double-escaped for quotation in source code.
var lstTests = [
'~/{Downloads,Pictures}/*.{jpg,gif,png}',
'It{{em,alic}iz,erat}e{d,}, please.',
'{,{,gotta have{ ,\\, again\\, }}more }cowbell!',
'{}} some }{,{\\\\{ edge, edge} \\,}{ cases, {here} \\\\\\\\\\}'
];
 
 
// 1. Return each expression with an indented list of its expansions, while
// 2. logging each parse tree to the console.log() stream
 
return lstTests.map(function (s) {
return s + '\n\n' + expansions(s).map(function (x) {
return ' ' + x;
}).join('\n');
}).join('\n\n');
 
})();

Value returned by function:

~/{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} \\\\\}

Sample of parse trees logged to the console:

{
"fn": "[function and]",
"args": [
"It",
{
"fn": "[function or]",
"args": [
{
"fn": "[function and]",
"args": [
{
"fn": "[function or]",
"args": [
"em",
"alic"
]
},
"iz"
]
},
"erat"
]
},
"e",
{
"fn": "[function or]",
"args": [
"d",
""
]
},
",",
" please."
]
}


Kotlin[edit]

Translation of: Java
// version 1.1.2
 
object BraceExpansion {
fun expand(s: String) = expandR("", s, "")
 
private val r = Regex("""([\\]{2}|[\\][,}{])""")
 
private fun expandR(pre: String, s: String, suf: String) {
val noEscape = s.replace(r, " ")
var sb = StringBuilder("")
var i1 = noEscape.indexOf('{')
var i2 = 0
 
outer@ while (i1 != -1) {
sb = StringBuilder(s)
var depth = 1
i2 = i1 + 1
while (i2 < s.length && depth > 0) {
val c = noEscape[i2]
if (c == '{')
depth++
else if (c == '}')
depth--
if (c == ',' && depth == 1)
sb[i2] = '\u0000'
else if (c == '}' && depth == 0 && sb.indexOf("\u0000") != -1)
break@outer
i2++
}
i1 = noEscape.indexOf('{', i1 + 1)
}
if (i1 == -1) {
if (suf.length > 0)
expandR(pre + s, suf, "")
else
println("$pre$s$suf")
}
else {
for (m in sb.substring(i1 + 1, i2).split('\u0000'))
expandR(pre + s.substring(0, i1), m, s.substring(i2 + 1) + suf)
}
}
}
 
fun main(args: Array<String>) {
val strings = arrayOf(
"""~/{Downloads,Pictures}/*.{jpg,gif,png}""",
"""It{{em,alic}iz,erat}e{d,}, please.""",
"""{,{,gotta have{ ,\, again\, }}more }cowbell!""",
"""{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}"""
)
for (s in strings) {
println()
BraceExpansion.expand(s)
}
}
Output:
~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png

Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.

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

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

Perl[edit]

Perl has a built-in glob function which does brace expansion, but it can't be used to solve this task because it also does shell-like word splitting, wildcard expansion, and tilde expansion at the same time. The File::Glob core module gives access to the more low-level bsd_glob function which actually supports exclusive brace expansion, however it does not comply with this task's specification when it comes to preserving backslashes and handling unbalanced brace characters.

So here is a manual solution that implements the specification precisely:

sub brace_expand {
my $input = shift;
my @stack = ([my $current = ['']]);
 
while ($input =~ /\G ((?:[^\\{,}]++ | \\(?:.|\z))++ | . )/gx) {
if ($1 eq '{') {
push @stack, [$current = ['']];
}
elsif ($1 eq ',' && @stack > 1) {
push @{$stack[-1]}, ($current = ['']);
}
elsif ($1 eq '}' && @stack > 1) {
my $group = pop @stack;
$current = $stack[-1][-1];
 
# handle the case of brace pairs without commas:
@{$group->[0]} = map { "{$_}" } @{$group->[0]} if @$group == 1;
 
@$current = map {
my $c = $_;
map { map { $c . $_ } @$_ } @$group;
} @$current;
}
else { $_ .= $1 for @$current; }
}
 
# handle the case of missing closing braces:
while (@stack > 1) {
my $right = pop @{$stack[-1]};
my $sep;
if (@{$stack[-1]}) { $sep = ',' }
else { $sep = '{'; pop @stack }
$current = $stack[-1][-1];
@$current = map {
my $c = $_;
map { $c . $sep . $_ } @$right;
} @$current;
}
 
return @$current;
}

Usage demonstration:

while (my $input = <DATA>) {
chomp($input);
print "$input\n";
print " $_\n" for brace_expand($input);
print "\n";
}
 
__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} \\\\\}

Perl 6[edit]

The task description allows the taking of shortcuts, but please note that we are not taking any shortcuts here. The solution is short because this particular problem happens to map quite naturally to the strengths of Perl 6.

First, the parsing is handled with a grammar that can backtrack in the few places this problem needs it. The + quantifier matches one or more alternatives (we handle the case of a single alternative in the walk now), and the % modifier requires a comma between each quantified item to its left. Note that the * quantifiers do not backtrack here, because the token keyword suppresses that; all the backtracking here fails over to a different alternative in an outer alternation (that is, things separated by the | character in the grammar. Most of these failovers just nibble an uninteresting character and continue.)

On the other end, we recursively walk the parse tree returning expanded sublists, and we do the cartesian concatenation of sublists at each level by use of the X~ operator, which is a "cross" metaoperator used on a simple ~ concatenation. As a list infix operator, X~ does not care how many items are on either side, which is just what you want in this case, since some of the arguments are strings and some are lists. Here we use a fold or reduction form in square brackets to interpose the cross-concat between each value generated by the map, which returns a mixture of lists and literal strings. One other thing that might not be obvious: if we bind to the match variable, $/, we automatically get all the syntactic sugar for its submatches. In this case, $0 is short for $/[0], and represents all the submatches captured by 0th set of parens in either TOP or alt. $<meta> is likewise short for $/<meta>, and retrieves what was captured by that named submatch.

grammar BraceExpansion {
token TOP { ( <meta> | . )* }
token meta { '{' <alts> '}' | \\ . }
token alts { <alt>+ % ',' }
token alt { ( <meta> | <-[ , } ]> )* }
}
 
sub crosswalk($/) {
[X~] '', $0.map: -> $/ { ([$<meta><alts><alt>.&alternatives]) or ~$/ }
}
 
sub alternatives($_) {
when :not { () }
when 1 { '{' X~ $_».&crosswalk X~ '}' }
default { $_».&crosswalk }
}
 
sub brace-expand($s) { crosswalk BraceExpansion.parse($s) }

And to test...

sub bxtest(*@s) {
for @s -> $s {
say "\n$s";
for brace-expand($s) {
say " ", $_;
}
}
}
 
bxtest Q:to/END/.lines;
~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some {\\{edge,edgy} }{ cases, here\\\}
a{b{1,2}c
a{1,2}b}c
a{1,{2},3}b
a{b{1,2}c{}}
more{ darn{ cowbell,},}
ab{c,d\,e{f,g\h},i\,j{k,l\,m}n,o\,p}qr
{a,{\,b}c
a{b,{{c}}
{a{\}b,c}d
{a,b{{1,2}e}f
END
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,edgy} }{ cases, here\\\}
    {}} some {\\edge }{ cases, here\\\}
    {}} some {\\edgy }{ cases, here\\\}

a{b{1,2}c
    a{b1c
    a{b2c

a{1,2}b}c
    a1b}c
    a2b}c

a{1,{2},3}b
    a1b
    a{2}b
    a3b

a{b{1,2}c{}}
    a{b1c{}}
    a{b2c{}}

more{ darn{ cowbell,},}
    more darn cowbell
    more darn
    more

ab{c,d\,e{f,g\h},i\,j{k,l\,m}n,o\,p}qr
    abcqr
    abd\,efqr
    abd\,eg\hqr
    abi\,jknqr
    abi\,jl\,mnqr
    abo\,pqr

{a,{\,b}c
    {a,{\,b}c

a{b,{{c}}
    a{b,{{c}}

{a{\}b,c}d
    {a\}bd
    {acd

{a,b{{1,2}e}f
    {a,b{1e}f
    {a,b{2e}f

Phix[edit]

Fairly straightforward recursive solution

--
-- demo\rosetta\Brace_expansion.exw
-- ================================
--
function pair(sequence stems, sequence brest)
sequence res = {}
for i=1 to length(stems) do
for j=1 to length(brest) do
res = append(res,stems[i]&brest[j])
end for
end for
return res
end function
 
function brarse(string s)
integer idx = 1
while idx<=length(s) do
integer ch = s[idx]
if ch='{' then
sequence alts = {idx}
idx += 1
integer l0 = idx
bool nest = false
integer level = 1
while idx<=length(s) do
switch s[idx] do
case '{': level += 1
nest = true
case '}': level -= 1
if level=0 then exit end if
case ',': if level=1 then
alts = append(alts,idx)
end if
case '\\': idx += 1
end switch
idx += 1
end while
if length(alts)>1 and level=0 then
alts &= idx
sequence stems = {}
string stem = s[1..alts[1]-1]
for i=2 to length(alts) do
string rest = s[alts[i-1]+1..alts[i]-1]
if nest then
sequence inners = brarse(rest)
for j=1 to length(inners) do
stems = append(stems,stem&inners[j])
end for
else
stems = append(stems,stem&rest)
end if
end for
return pair(stems,brarse(s[idx+1..$]))
elsif nest then
return pair({s[1..l0-1]},brarse(s[l0..$]))
end if
end if
idx += 1
end while
return {s}
end function
 
-- (since ? and pp() add their own backslash escapes:)
procedure edump(sequence x)
for i=1 to length(x) do
printf(1,"%s\n",{x[i]})
end for
end procedure
 
edump(brarse("~/{Downloads,Pictures}/*.{jpg,gif,png}"))
edump(brarse("It{{em,alic}iz,erat}e{d,}, please."))
edump(brarse(`{,{,gotta have{ ,\, again\, }}more }cowbell!`))
edump(brarse(`{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}`))
Output:
~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png
Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.
cowbell!
more cowbell!
gotta have more cowbell!
gotta have\, again\, more cowbell!
{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}
{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}

PicoLisp[edit]

(de braceExpand (Str)
(let Lst
(make
(for (Lst (chop Str) Lst)
(case (pop 'Lst)
("\\" (link (pop 'Lst)))
("{"
(recur ()
(let L
(make
(while
(case (pop 'Lst)
("\\" (link (pop 'Lst)) Lst)
("{" (recurse) Lst)
("}" NIL)
("," (link 0) Lst) # Replace commata with '0'
(T (link @) Lst) ) ) )
(if (= "}" @) # Was closing brace
(if (member 0 L) # Has comma(ta)
(link (split L 0))
(chain (list "{") (replace L 0 ",") (list "}")))
(chain (list "{") (replace L 0 ",")) ) ) ) )
(T (link @)) ) ) )
(recur (Lst)
(ifn (find pair Lst)
(list (pack Lst))
(let R (recurse (cdr Lst))
(mapcan
'((A) (mapcar '((B) (pack A B)) R))
(if (pair (car Lst))
(mapcan recurse (car Lst))
(list (car Lst)) ) ) ) ) ) ) )

Test:

(test
(quote
"~/Downloads/*.jpg"
"~/Downloads/*.gif"
"~/Downloads/*.png"
"~/Pictures/*.jpg"
"~/Pictures/*.gif"
"~/Pictures/*.png" )
(braceExpand "~/{Downloads,Pictures}/*.{jpg,gif,png}") )
 
(test
(quote
"Itemized, please."
"Itemize, please."
"Italicized, please."
"Italicize, please."
"Iterated, please."
"Iterate, please." )
(braceExpand "It{{em,alic}iz,erat}e{d,}, please.") )
 
(test
(quote
"cowbell!"
"more cowbell!"
"gotta have more cowbell!"
"gotta have\, again\, more cowbell!" )
(braceExpand "{,{,gotta have{ ,\\, again\\, }}more }cowbell!") )
 
(test
(quote
"{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}"
"{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}" )
(braceExpand "{}} some }{,{\\\\{ edge, edge} \\,}{ cases, {here} \\\\\\\\\\}") )

PowerShell[edit]

Works with: PowerShell version 2
 
function Expand-Braces ( [string]$String )
{
$Escaped = $False
$Stack = New-Object System.Collections.Stack
$ClosedBraces = $BracesToParse = $Null
 
ForEach ( $i in 0..($String.Length-1) )
{
Switch ( $String[$i] )
{
'\' {
$Escaped = -not $Escaped
break
}
 
'{' {
If ( -not $Escaped ) { $Stack.Push( [pscustomobject]@{ Delimiters = @( $i ) } ) }
}
 
',' {
If ( -not $Escaped -and $Stack.Count ) { $Stack.Peek().Delimiters += $i }
}
 
'}' {
If ( -not $Escaped -and $Stack.Count )
{
$Stack.Peek().Delimiters += $i
$ClosedBraces = $Stack.Pop()
If ( $ClosedBraces.Delimiters.Count -gt 2 )
{
$BracesToParse = $ClosedBraces
}
}
}
 
default { $Escaped = $False }
}
}
 
If ( $BracesToParse )
{
$Start = $String.Substring( 0, $BracesToParse.Delimiters[0] )
$End = $String.Substring( $BracesToParse.Delimiters[-1] + 1 )
 
ForEach ( $i in 0..($BracesToParse.Delimiters.Count-2) )
{
$Option = $String.Substring( $BracesToParse.Delimiters[$i] + 1, $BracesToParse.Delimiters[$i+1] - $BracesToParse.Delimiters[$i] - 1 )
Expand-Braces ( $Start + $Option + $End )
}
}
Else
{
$String
}
}
 
 
$TestStrings = @(
'It{{em,alic}iz,erat}e{d,}, please.'
'~/{Downloads,Pictures}/*.{jpg,gif,png}'
'{,{,gotta have{ ,\, again\, }}more }cowbell!'
'{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}'
)
 
ForEach ( $String in $TestStrings )
{
''
$String
'------'
Expand-Braces $String
}
 
Output:
It{{em,alic}iz,erat}e{d,}, please.
------
Itemized, please.
Italicized, please.
Iterated, please.
Itemize, please.
Italicize, please.
Iterate, please.

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

{,{,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} \\\\\}

Python[edit]

def getitem(s, depth=0):
out = [""]
while s:
c = s[0]
if depth and (c == ',' or c == '}'):
return out,s
if c == '{':
x = getgroup(s[1:], depth+1)
if x:
out,s = [a+b for a in out for b in x[0]], x[1]
continue
if c == '\\' and len(s) > 1:
s, c = s[1:], c + s[1]
 
out, s = [a+c for a in out], s[1:]
 
return out,s
 
def getgroup(s, depth):
out, comma = [], False
while s:
g,s = getitem(s, depth)
if not s: break
out += g
 
if s[0] == '}':
if comma: return out, s[1:]
return ['{' + a + '}' for a in out], s[1:]
 
if s[0] == ',':
comma,s = True, s[1:]
 
return None
 
# stolen cowbells from perl6 example
for s in '''~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\\\{ edge, edge} \,}{ cases, {here} \\\\\\\\\}'''
.split('\n'):
print "\n\t".join([s] + getitem(s)[0]) + "\n"
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} \\\\\}

Racket[edit]

Translation of: Python
#lang racket/base
(require racket/match)
(define (merge-lists as . bss)
(match bss
['() as]
[(cons b bt)
(apply merge-lists
(for*/list ((a (in-list as)) (b (in-list b))) (append a b))
bt)]))
 
(define (get-item cs depth)
(let loop ((out '(())) (cs cs))
(match cs
['() (values out cs)]
[`(,(or #\, #\}) . ,more) #:when (positive? depth) (values out cs)]
[`(#\{ . ,more)
(=> no-group-match)
(define-values (group-out group-rem) (get-group more (add1 depth) no-group-match))
(loop (merge-lists out group-out) group-rem)]
[(or `(#\\ ,c ,more ...) `(,c . ,more)) (loop (merge-lists out (list (list c))) more)])))
 
(define (get-group cs depth no-group-match)
(let loop ((out '()) (cs cs) (comma? #f))
(when (null? cs) (no-group-match))
(define-values (item-out item-rem) (get-item cs depth))
(when (null? item-rem) (no-group-match))
(let ((out (append out item-out)))
(match item-rem
[`(#\} . ,more) #:when comma? (values out more)]
[`(#\} . ,more) (values (merge-lists '((#\{)) out '((#\}))) more)]
[`(#\, . ,more) (loop out more #t)]
[_ (no-group-match)]))))
 
(define (brace-expand s)
(let-values (([out rem] (get-item (string->list s) 0))) (map list->string out)))
 
(module+ test
(define patterns
(list
"~/{Downloads,Pictures}/*.{jpg,gif,png}"
"It{{em,alic}iz,erat}e{d,}, please."
#<<P
{,{,gotta have{ ,\, again\, }}more }cowbell!
P
#<<P
{}} some }{,{\\\\{ edge, edge} \,}{ cases, {here} \\\\\\\\\}
P
))
(for ((s (in-list patterns)) #:when (printf "expand: ~a~%" s) (x (in-list (brace-expand s))))
(printf "\t~a~%" x)))
Output:
expand: ~/{Downloads,Pictures}/*.{jpg,gif,png}
	~/Downloads/*.jpg
	~/Downloads/*.gif
	~/Downloads/*.png
	~/Pictures/*.jpg
	~/Pictures/*.gif
	~/Pictures/*.png
expand: It{{em,alic}iz,erat}e{d,}, please.
	Itemized, please.
	Itemize, please.
	Italicized, please.
	Italicize, please.
	Iterated, please.
	Iterate, please.
expand: {,{,gotta have{ ,\, again\, }}more }cowbell!
	cowbell!
	more cowbell!
	gotta have more cowbell!
	gotta have, again, more cowbell!
expand: {}} some }{,{\\\\{ edge, edge} \,}{ cases, {here} \\\\\\\\\}
	{}} some }{,{\\ edge ,}{ cases, {here} \\\\}
	{}} some }{,{\\ edge ,}{ cases, {here} \\\\}

REXX[edit]

/*---------------------------------------------------------------------
* Brace expansion
* 26.07.2016
* s.* holds the set of strings
*--------------------------------------------------------------------*/

text.1='{,{,gotta have{ ,\, again\, }}more }cowbell!'
text.2='~/{Downloads,Pictures}/*.{jpg,gif,png}'
text.3='It{{em,alic}iz,erat}e{d,}, please. '
text.4='{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\} '
text.5='x{,a,b,c}{d,e}y'
text.6='aa{,{,11}cc}22'
text.7='{}'
Parse Arg dbg
oid='brace.xxx'; 'erase' oid
Do case=1 To 7
Call brac text.case
End
Return
brac:
s.=0
Parse Arg s
Say ''
Say ' 's
s.1.0=1 /* first iteration */
s.1.1=s /* the initial string */
Do it=1 To 10 Until todo=0 /* Iterate until all done */
todo=0 /* will be 1 if more to be done */
Call dbg 'Iteration' it
do di=1 To s.it.0 /* show current state */
Call dbg 's.'it'.'di s.it.di
End
ita=it+1 /* index for next set of strings*/
xp=0
do di=1 To s.it.0 /* loop over all strings */
Call dbg it'.'di s.it.di
Call bra s.it.di /* analyze current string */
If braces=1 Then Do /* If brace groups were found */
Do bgi=1 To bgdata.0 /* loop over grace groups */
If end.bgi=0 Then Iterate /* Incomplete bg (... ) */
clist=''
Do cj=1 To ci.bgi.0
clist=clist ci.bgi.cj
End
Call dbg bgdata.bgi '->' clist
If ccount.bgi>0 Then Do /* comma(s) founf in bg */
Call expand bgi /* expand this bg */
xp=1 /* indicate that we worked */
Leave
End
End
If xp=0 Then Do /* nothing done */
z=s.ita.0+1 /* copy string to next iteration*/
s.ita.z=s.it.di
End
End
Else Do /* no brace group */
z=s.ita.0+1 /* copy string to next iteration*/
s.ita.z=s
s.ita.0=z
End
End
Do dd=1 To s.ita.0 /* log current set of strings */
Call dbg ita dd s.ita.dd
End
End
Do dd=1 To s.it.0 /* show final set of strings */
Say dd s.it.dd
End
Return
 
bra:
/*---------------------------------------------------------------------
* Analyze the given string
* Input: s
* Output:
* bgdata.* Array of data about brace groups:
* level start column comma positions end column
*--------------------------------------------------------------------*/

parse Arg s
Call dbg 'bra:' s
level=0
bgdata.=0
bgn=0
bgnmax=0
ccount.=0
ol=''
ci.=0
bgnl=''
braces=0
end.=0
escape=0
Do i=1 To length(s)
c=substr(s,i,1)
Select
When escape Then
escape=0
When c='\' Then
escape=1
When c='{' Then Do
level=level+1
Call bm c
co=level
End
When c='}' Then Do
If level>0 Then Do
co=level
Call bm c
level=level-1
End
End
When c=',' Then Do
co=level
If co>0 Then Do
ccount.bgn=ccount.bgn+1
z=ccount.bgn
ci.bgn.0=z
ci.bgn.z=i
End
If ccount.bgn>0 Then
braces=1
End
Otherwise
co=level
End
ol=ol||co
bgnl=bgnl||bgn
End
Call dbg s
Call dbg ol
Call dbg left(copies('123456789.',10),length(s))
Call dbg bgnl
Do bgi=1 To bgdata.0
If end.bgi=1 Then Do
cl=''
Do cii=1 To ci.bgi.0
cl=cl ci.bgi.cii
End
Parse Var bgdata.bgi level a e
Call dbg bgi level a cl e
End
End
Return
 
bm:
/*---------------------------------------------------------------------
* Brace Management
* for '{' create a new brace group )record level and start column
* for '}' look for corresponding bg and add end column
* Input: column and character ( '{' or '}' )
* Output: bgdata.* level start-column [end-column]
*--------------------------------------------------------------------*/

Parse Arg oc
Call dbg oc i level
If oc='{' Then Do
z=bgdata.0+1
bgdata.z=level i
bgdata.0=z
bgn=bgnmax+1
bgnmax=bgn
End
Else Do
Do bgi=bgdata.0 To 1 By -1
If level=word(bgdata.bgi,1) Then Do
bgdata.bgi=bgdata.bgi i
end.bgi=1
Leave
End
End
bgn=bgn-1
Call dbg bgdata.bgi 'bgn='bgn
End
Return
 
expand:
/*---------------------------------------------------------------------
* Expand a brace group in string s
*--------------------------------------------------------------------*/

Parse Arg bgi
Parse Var bgdata.bgi . start end
clist=start clist end
If words(clist)>0 Then Do /* commas in brace group */
left=left(s,start-1) /* part of s before the '{' */
rite=substr(s,end+1) /* part of s after the '}' */
Do k=1 To words(clist)-1 /* Loop over comma positions */
a=word(clist,k) /* start position */
e=word(clist,k+1) /* end position */
choice.k=substr(s,a+1,e-a-1) /* one of the choices */
z=s.ita.0+1 /* add new string to next set */
s.ita.z=left||choice.k||rite /* construct new string */
s.ita.0=z
todo=1 /* maybe more to be done */
End
End
Else Do /* no commas */
z=s.ita.0+1 /* copy string as is to next set*/
s.ita.z=s
s.ita.0=z
End
Do zz=1 To s.ita.0
Call dbg zz s.ita.zz
End
Return
 
dbg: /* handle debug output */
If dbg<>'' Then /* argument given */
Say arg(1) /* show on screen */
Call lineout oid,arg(1) /* write to file */
Return
Output:
J:\>rexx braces

  {,{,gotta have{ ,\, again\, }}more }cowbell!
1 cowbell!
2 more cowbell!
3 gotta have more cowbell!
4 gotta have\, again\, more cowbell!

  ~/{Downloads,Pictures}/*.{jpg,gif,png}
1 ~/Downloads/*.jpg
2 ~/Downloads/*.gif
3 ~/Downloads/*.png
4 ~/Pictures/*.jpg
5 ~/Pictures/*.gif
6 ~/Pictures/*.png

  It{{em,alic}iz,erat}e{d,}, please.
1 Itemized, please.
2 Itemize, please.
3 Italicized, please.
4 Italicize, please.
5 Iterated, please.
6 Iterate, please.

  {}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
1 {}} some }{,{\\ edge \,}{ cases, {here} \\\\\}
2 {}} some }{,{\\ edge \,}{ cases, {here} \\\\\}

  x{,a,b,c}{d,e}y
1 xdy
2 xey
3 xady
4 xaey
5 xbdy
6 xbey
7 xcdy
8 xcey

  aa{,{,11}cc}22
1 aa22
2 aacc22
3 aa11cc22

  {}
1 {}

Ruby[edit]

Translation of: Python
def getitem(s, depth=0)
out = [""]
until s.empty?
c = s[0]
break if depth>0 and (c == ',' or c == '}')
if c == '{' and x = getgroup(s[1..-1], depth+1)
out = out.product(x[0]).map{|a,b| a+b}
s = x[1]
else
s, c = s[1..-1], c + s[1] if c == '\\' and s.size > 1
out, s = out.map{|a| a+c}, s[1..-1]
end
end
return out, s
end
 
def getgroup(s, depth)
out, comma = [], false
until s.empty?
g, s = getitem(s, depth)
break if s.empty?
out += g
case s[0]
when '}' then return (comma ? out : out.map{|a| "{#{a}}"}), s[1..-1]
when ',' then comma, s = true, s[1..-1]
end
end
end
 
strs = <<'EOS'
~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
EOS
 
strs.each_line do |s|
puts s.chomp!
puts getitem(s)[0].map{|str| "\t"+str}
puts
end
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} \\\\\}

Scala[edit]

Works with: Scala version 2.11
 
import collection.mutable.ListBuffer
case class State(isChild: Boolean, alts: ListBuffer[String], rem: List[Char])
def expand(s: String): Seq[String] = {
def parseGroup(s: State): State = s.rem match {
case Nil => s.copy(alts = ListBuffer("{" + s.alts.mkString(",")))
case ('{' | ',')::sp =>
val newS = State(true, ListBuffer(""), rem = sp)
val elem = parseElem(newS)
elem.rem match {
case Nil => elem.copy(alts = elem.alts.map(a => "{" + s.alts.map(_ + ",").mkString("") + a))
case elemrem => parseGroup(s.copy(alts = (s.alts ++= elem.alts), rem = elem.rem))
}
case '}'::sp =>
if (s.alts.isEmpty) s.copy(alts = ListBuffer("{}"), rem = sp)
else if(s.alts.length == 1) s.copy(alts = ListBuffer("{"+s.alts.head + "}"), rem = sp)
else s.copy(rem = sp)
case _ => throw new Exception("parseGroup should be called only with delimitors")
}
def parseElem(s: State): State = s.rem match {
case Nil => s
case '{'::sp =>
val ys = parseGroup(State(true, ListBuffer(), s.rem))
val newAlts = for { x <- s.alts; y <- ys.alts} yield x + y
parseElem(s.copy(alts = newAlts, rem = ys.rem))
case (',' | '}')::_ if s.isChild => s
case '\\'::c::sp => parseElem(s.copy(alts = s.alts.map(_ + '\\' + c), rem = sp))
case c::sp => parseElem(s.copy(alts = s.alts.map(_ + c), rem = sp))
}
parseElem(State(false, ListBuffer(""), s.toList)).alts
}
 

Demonstrating:

 
println(expand("""~/{Downloads,Pictures}/*.{jpg,gif,png}""") mkString "\n")
println(expand("It{{em,alic}iz,erat}e{d,}, please.") mkString "\n")
println(expand("""{,{,gotta have{ ,\, again\, }}more }cowbell!""") mkString "\n")
println(expand("""{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}""") mkString "\n")
 
Output:
~/Downloads/*.jpg
~/Downloads/*.gif
~/Downloads/*.png
~/Pictures/*.jpg
~/Pictures/*.gif
~/Pictures/*.png
Itemized, please.
Itemize, please.
Italicized, please.
Italicize, please.
Iterated, please.
Iterate, please.
cowbell!
more cowbell!
gotta have more cowbell!
gotta have\, again\, more cowbell!
{}} some }{,\\ edge \,{ cases, {here} \\\\\}
{}} some }{,\\ edge \,{ cases, {here} \\\\\}

Tcl[edit]

Works with: Tcl version 8.6
package require Tcl 8.6
 
proc combine {cases1 cases2 {insert ""}} {
set result {}
foreach c1 $cases1 {
foreach c2 $cases2 {
lappend result $c1$insert$c2
}
}
return $result
}
proc expand {string *expvar} {
upvar 1 ${*expvar} expanded
set a {}
set result {}
set depth 0
foreach token [regexp -all -inline {(?:[^\\{},]|\\.)+|[\\{},]} $string] {
switch $token {
"," {
if {$depth == 0} {
lappend result {*}[commatize $a]
set a {}
set expanded 1
continue
}
}
"\{" {incr depth 1}
"\}" {incr depth -1}
}
append a $token
}
lappend result {*}[commatize $a]
return $result
}
proc commatize {string} {
set current {{}}
set depth 0
foreach token [regexp -all -inline {(?:[^\\{},]|\\.)+|[\\{},]} $string] {
switch $token {
"\{" {
if {[incr depth] == 1} {
set collect {}
continue
}
}
"\}" {
if {[incr depth -1] == 0} {
set foundComma 0
set exp [expand $collect foundComma]
if {!$foundComma} {
set exp [lmap c [commatize $collect] {set c \{$c\}}]
}
set current [combine $current $exp]
continue
} elseif {$depth < 0} {
set depth 0
}
}
}
if {$depth} {
append collect $token
} else {
set current [lmap s $current {set s $s$token}]
}
}
if {$depth} {
set current [combine $current [commatize $collect] "\{"]
}
return $current
}

Demonstrating:

foreach testcase {
"~/{Downloads,Pictures}/*.{jpg,gif,png}"
"It{{em,alic}iz,erat}e{d,}, please."
"{,{,gotta have{ ,\\, again\\, }}more }cowbell!"
"\{\}\} some \}\{,\{\\\\\{ edge, edge\} \\,\}\{ cases, \{here\} \\\\\\\\\\\}"
} {
puts $testcase\n\t[join [commatize $testcase] \n\t]
}
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} \\\\\}

zkl[edit]

This is a two pass algorithm (2*length(string)), one pass to find valid {} pairs, the next pass to expand them.

fcn eyeball(code,ps=L(),brace=False){ //-->indexes of valid braces & commas
cs:=L();
foreach c in (code){ // start fresh or continue (if recursing)
switch(c){
case("\\"){ __cWalker.next(); }
case(",") { if(brace) cs.append(__cWalker.n); } // maybe valid
case("{") { // this is real only if there is matching } and a comma
n:=__cWalker.n;
_,cz:=self.fcn(__cWalker,ps,True);
if(cz){ ps.append(n,__cWalker.n); ps.extend(cz) } // valid {} pair
}
case("}"){ if(brace) return(ps,cs); }
}
}
return(ps,False)
}
 
fcn expando(code,strings=T("")){
reg [const] stack=List(); reg roots,cs; bs,_:=eyeball(code);
foreach c in (code){
if(bs.holds(__cWalker.n)){
if (c=="{") { stack.append(cs); cs=0; roots=strings; }
else if(c==",") { stack.append(strings); strings=roots; cs+=1; }
else if(c=="}") { do(cs){ strings=stack.pop().extend(strings); } cs=stack.pop(); }
}else if(c=="\\"){
c="\\"+__cWalker.next();
strings=strings.apply('+(c));
}
else strings=strings.apply('+(c));
}
strings
}
foreach bs in (T("It{{em,alic}iz,erat}e{d,}",  "~/{Downloads,Pictures}/*.{jpg,gif,png}",
"It{{em,alic}iz,erat}e{d,}, please.", "a{2,1}b{X,Y,X}c", 0'|a\\{\\\{b,c\,d}|,
"{a,b{c{,{d}}e}f", 0'|{,{,gotta have{ ,\, again\, }}more }cowbell!|,
0'|{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}|))
{
"%s expands to\n  %s".fmt(bs,expando(bs)).println();
}
Output:
It{{em,alic}iz,erat}e{d,} expands to
   L("Itemized","Italicized","Iterated","Itemize","Italicize","Iterate")
~/{Downloads,Pictures}/*.{jpg,gif,png} expands to
   L("~/Downloads/*.jpg","~/Pictures/*.jpg","~/Downloads/*.gif","~/Pictures/*.gif","~/Downloads/*.png","~/Pictures/*.png")
It{{em,alic}iz,erat}e{d,}, please. expands to
   L("Itemized, please.","Italicized, please.","Iterated, please.","Itemize, please.","Italicize, please.","Iterate, please.")
a{2,1}b{X,Y,X}c expands to
   L("a2bXc","a1bXc","a2bYc","a1bYc","a2bXc","a1bXc")
a\\{\\\{b,c\,d} expands to
   L("a\\\\\{b","a\\c\,d")
{a,b{c{,{d}}e}f expands to
   L("{a,b{ce}f","{a,b{c{d}e}f")
{,{,gotta have{ ,\, again\, }}more }cowbell! expands to
   L("cowbell!","more cowbell!","gotta have more cowbell!","gotta have\, again\, more cowbell!")
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\} expands to
   L("{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}","{}} some }{,{\\ edge \,}{ cases, {here} \\\\\}")