Bracket Expansion!

Question

Your challenge is to expand some brackets in a program's input as shown:

1. Find a string s between two matching brackets [ and ], with a single digit n after the closing bracket.
2. Remove the brackets.
3. Replace s with itself repeated n times. (If n is 0, simply remove s.)
4. Go to step 1 until there are no more matching brackets in the input.

Additional rules and clarifications:

• You will take input and give output via any allowed means.
• A trailing newline in the output is allowed.
• You only need to handle printable ASCII in the input.
• You may assume that all brackets match, i.e. you will never receive the input []]]] or [[[[].
• You may assume that each closing bracket ] has a digit after it.

Test cases:

Input                -> Output
[Foo[Bar]3]2         -> FooBarBarBarFooBarBarBar
[one]1[two]2[three]3 -> onetwotwothreethreethree
[three[two[one]1]2]3 -> threetwoonetwoonethreetwoonetwoonethreetwoonetwoone
[!@#[$%^[&*(]2]2]2   -> !@#$%^&*(&*($%^&*(&*(!@#$%^&*(&*($%^&*(&*(
[[foo bar baz]1]1    -> foo bar baz
[only once]12        -> only once2
[only twice]23456789 -> only twiceonly twice3456789
[remove me!]0        -> 
before [in ]2after   -> before in in after

As this is code-golf, the shortest answer in each language wins. Good luck!

Answer 1

Gema, 17 characters

[#]?=@repeat{?;#}

Sample run:

bash-4.4$ gema '[#]?=@repeat{?;#}' <<< '[three[two[one]1]2]3'
threetwoonetwoonethreetwoonetwoonethreetwoonetwoone

Answer 2

Retina, 24 23 22 bytes

+`\[([^][]*)](.)
$2*$1

Try it online! This is practically a builtin in Retina 1. Edit: Saved 1 byte thanks to @Kobi. 47 45 bytes in Retina 0.8.2:

].
]$&$*¶
{+`\[([^][]*)]¶
$1[$1]
\[([^][]*)]

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Answer 3

Haskell, 101 96 bytes

fst.(""%)
infix 4%
s%']':d:r=(['1'..d]>>s,r)
s%'[':r|(t,q)<-""%r=s++t%q
s%x:r=s++[x]%r
s%e=(s,e)

Try it online! Instead of using regular expression like most of the other answers, this implements a recursive parser.

-5 bytes thanks to BMO!

Answer 4

Perl 5, 34 33 29 + 1 (-p) = 30 bytes

s/.([^[]*?)](.)/$1x$2/e&&redo

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Cut it down with some help from @Shaggy and @TonHospel.

Answer 5

Japt v2, 21 20 19 bytes

Saved 2 bytes thanks to @Shaggy

e/.([^[]*?)](./@YpZ

Test it online!

e is recursive replace, which makes one replacement at a time until there are no more matches. In this case, matches of the regex /\[([^[]*?)](\d)/g are replaced with with <inner text> repeated <digit> times until there are no more matches.

According to what I have planned (here), this regex should eventually be at least 3 2 bytes shorter:

‹[“⁽[»₋”]“.›

Answer 6

JavaScript, 71 67 66 bytes

I had a 54 byte solution but it got screwed over by the second test case! :(

f=s=>s!=(x=s.replace(/.([^[]*?)](.)/,(_,y,z)=>y.repeat(z)))?f(x):x

---

Test Cases

f=s=>s!=(x=s.replace(/.([^[]*?)](.)/,(_,y,z)=>y.repeat(z)))?f(x):x
o.innerText=`[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`.split`\n`.map(x=>x.padEnd(22)+`:  `+f(x)).join`\n`

<pre id=o></pre>

Answer 7

Python 3, 110 93 92 bytes

import re
f=lambda s:f(re.sub(r'\[([^][]+)\](.)',lambda m:m[1]*int(m[2]),s))if'['in s else s

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-17 bytes thanks to pizzapants184 -1 byte thanks to Kevin Cruijssen

Answer 8

Scala, 173 bytes

l.foreach{x=>def r(c:String):String={val t="""\[([^\[\]]*)\](.)""".r.unanchored;c match{case t(g,h)=>r(c.replaceAllLiterally(s"[$g]$h",g*h.toInt));case _=>c}};println(r(x))}

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Expanded:

l.foreach { x =>
  def remove(current: String): String = {
    val test ="""\[([^\[\]]*)\](.)""".r.unanchored
    current match {
      case test(g, h) => remove(current.replaceAllLiterally(s"[$g]$h", g * h.toInt))
      case _ => current
    }
  }

  println(remove(x))
}

Old solution

Scala, 219 215 213 212 199 bytes

l.foreach{x=>def r(c:String):String={"""\[([^\[\]]*)\](.)""".r.findFirstMatchIn(c).map{x=>val g=x.group(1);val h=x.group(2).toInt;r(c.replaceAllLiterally(s"[$g]$h",g*h))}.getOrElse(c)};println(r(x))}

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Expanded:

l.foreach { x =>
  def remove(current: String): String = {
    """\[([^\[\]]*)\](.)""".r.findFirstMatchIn(current).map { x =>
      val g = x.group(1)
      val h = x.group(2).toInt
      remove(current.replaceAllLiterally(s"[$g]$h", g * h))
    }.getOrElse(current)
  }
  println(remove(x))
}

Where l is the list of strings that we will process.

Thanks Kevin Cruijssen for -1 byte

Went from 212 to 199 by removing an unused parameter, didn't pay attention.

Answer 9

Stacked, 39 38 bytes

Saved 1 byte thanks to Shaggy, golfed the regex!

['\[([^[\]]+)](.)'{.y x:x#~y*}recrepl]

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Simply recursively replaces a regex '\[([^[\]]+)](.)' with the repetition rule.

Answer 10

JavaScript - 77 75 72 bytes

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

Edit: updated regex with Shaggy's recommendation

Snippet:

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">

Answer 11

Python 3, 155 148 101 97 bytes

def f(x):
 a=x.rfind('[')
 if~a:b=x.find(']',a);x=f(x[:a]+x[a+1:b]*int(x[b+1])+x[b+2:])
 return x

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Thanks to HyperNeutrino and Mego for -47 bytes and user202729 for -4 bytes.

Answer 12

Java 8, 250 249 241 239 bytes

s->{for(;s.contains("[");)for(int i=0,j,k;i<s.length();)if(s.charAt(i++)==93){String t="",r=t;for(j=k=s.charAt(i)-48;j-->0;)t+=s.replaceAll(r="(.*)\\[([^\\]]+)\\]"+k+"(.*)","$2");s=k<1?t:s.replaceFirst(r,"$1$3").replace("",t);}return s;}

-2 bytes thanks to @JonathanFrech (code contains two unprintable ASCII characters now, which can be seen in the TIO-link below).

Sigh... Java with regex is so damn limited.. I'll just quote myself from another answer here:

Replacing WWWW with 222W is easy in Java, but with 4W not.. If only Java had a way to use the regex capture-group for something.. Getting the length with "$1".length(), replacing the match itself with "$1".replace(...), converting the match to an integer with new Integer("$1"), or using something similar as Retina (i.e. s.replaceAll("(?=(.)\\1)(\\1)+","$#2$1")) or JavaScript (i.e. s.replaceAll("(.)\\1+",m->m.length()+m.charAt(0))) would be my number 1 thing I'd like to see in Java in the future to benefit codegolfing.. >.> I think this is the 10th+ time I hate Java can't do anything with the capture-group match..
Quote from here.

Explanation:

Try it online.

s->{                           // Method with String as both parameter and return-type
  for(;s.contains("[");)       //  Loop as long as the String contains a block-bracket
    for(int i=0,j,k;i<s.length();)
                               //   Inner loop over the characters of the String
      if(s.charAt(i++)==93){   //    If the current character is a closing block-bracket:
        String t="",r=t;       //     Create two temp-Strings, starting empty
        for(j=k=s.charAt(i)-48;//     Take the digit after the closing bracket
            j-->0;)            //     Loop that many times:
          t+=s.replaceAll(r="(.*)\\[([^\\]]+)\\]"+k+"(.*)","$2");
                               //      Append `t` with the word inside the block brackets
        s=k<1?                 //     If the digit was 0:
           t                   //      Replace the input with an empty String as well
          :                    //     Else:
           s.replaceFirst(r,"$1$3").replace("",t);}
                               //      Replace the word between brackets by `t`,
                               //      and remove the digit
  return s;}                   //  Return the modified input-String as result

Answer 13

QuadR with the ≡ argument, 30 28 bytes

\[[^[]+?].
∊(⍎⊃⌽⍵M)⍴⊂1↓¯2↓⍵M

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\[[^[]+?]. replace "[non-[ stuff]character" with

¯2↓⍵M drop the last two characters of the Match ("]digit")
1↓ drop the first character ("[")
⊂ enclose to be treated as a whole
(…)⍴ reshape to length:
 ⌽⍵M reverse the Match
 ⊃ pick the first (the digit)
 ⍎ evaluate
∊ ϵnlist (flatten)

≡ repeat until no more changes happen

---

The equivalent Dyalog APL function is 47 bytes:

'\[[^[]+?].'⎕R{∊(⍎⊃⌽⍵.Match)⍴⊂1↓¯2↓⍵.Match}⍣≡

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Answer 14

C, 407 368 bytes

Thanks to Jonathan Frech for saving bytes.

golfed (file bracket.c):

i,j,k,l,n;char*f(a,m)char*a;{for(i=0;a[i];++i){a[i]==91&&(j=i+1);if(a[i]==93){k=a[i+1]-48;if(!k){for(l=i+2;l<m;)a[++l-i+j-4]=a[l];a=realloc(a,m-3);return f(a,m-3);}for(l=j;l<i;)a[~-l++]=a[l];for(l=i+2;l<m;)a[++l-4]=a[l];m-=3;n=m+~-k*(i---j--);a=realloc(a,n);for(l=i;l<m;)a[l+++~-k*(i-j)]=a[l];for(m=0;m<k;++m)for(l=j;l<i;)a[l+++m*(i-j)]=a[l];return f(a,n);}}return a;}

ungolfed with program:

#include <stdlib.h>
#include <stdio.h>

// '[' = 133
// ']' = 135
// '0' = 48

i, j, k, l, n;

char* f(a,m) char*a;
{
  for (i=0; a[i]; ++i) {
    a[i]==91&&(j=i+1);

    if (a[i]==93) {
      k=a[i+1]-48;

      if (!k) {
        for (l=i+2; l<m; )
          a[++l-i+j-4] = a[l];

        a = realloc(a,m-3);
        return f(a,m-3);
      }
      for (l=j;l<i;)
        a[~-l++] = a[l];
      for (l=i+2; l<m; )
        a[++l-4] = a[l];
      m -= 3;
      n = m+~-k*(i---j--);
      a = realloc(a,n);

      for (l=i; l<m; )
        a[l+++~-k*(i-j)] = a[l];
      for (m=0; m<k; ++m)
        for (l=j; l<i;)
          a[l+++m*(i-j)] = a[l];

      return f(a,n);
    }
  }
  return a;
}

int main()
{
  char c[]="[Foo[Bar]3]2";
  char *b;

  char cc[]="[remove me!]0";
  char *bb;

  char ccc[]="[only once]12";
  char *bbb;

  b=malloc(13);
  bb=malloc(14);
  bbb=malloc(14);

  for (i=0; i<13; ++i)
    b[i] = c[i];

  for (i=0; i<14; ++i)
    bb[i] = cc[i];

  for (i=0; i<14; ++i)
    bbb[i]=ccc[i];

  printf("%s\n", f(b, 13));
  printf("%s\n", f(bb, 14));
  printf("%s\n", f(bbb, 14));

  return 0;
}

Compiled with gcc 5.4.1, gcc bracket.c

Answer 15

Red, 147 bytes

f: func[t][a: charset[not"[]"]while[parse t[any a some[remove["["copy h any a"]"copy d a](insert/dup v: copy""h to-integer d)insert v | skip]]][]t]

Ungolfed:

f: func [t][
    a: charset [not "[]"]                          ; all chars except [ and ]
    while [ parse t [                              ; repeat while parse is returning true
        any a                                      ; 0 or more chars other than [ and ]
        some [                                     ; one or more block:
            remove ["[" copy h any a "]" copy d a] ; remove the entire block, store the
                                                   ; substring between the [] in h,
                                                   ; the digit into d
            (insert/dup v: copy "" h to-integer d) ; makes d copies of h 
            insert v                               ; and inserts them in place 
            | skip ]                               ; skip if no match
        ]                                       
    ][]                                            ; empty block for 'while'
    t                                              ; return the modified string
]

I started learning Red's Parse dialect only yesterday, so I'm sure my code can be improved further. Parse is incomparably more verbose than regex, but is very clear, flexible and readable and can be freely mixed with the rest of the Red language.

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Answer 16

Wolfram Language (Mathematica), 86 bytes

#//.s_:>StringReplace[s,"["~~x:Except["["|"]"]...~~"]"~~d_:>""<>x~Table~FromDigits@d]&

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Answer 17

Jelly, 30 bytes

œṡ”]µḢUœṡ”[ẋ€1¦Ṫ©Ḣ$FṚ;®
Çċ”]$¡

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---

Explanation.

œṡ”]µḢUœṡ”[ẋ€1¦Ṫ©Ḣ$FṚ;®    Helper link 1, expand once.
                           Assume input = "ab[cd]2ef".

œṡ      Split at first occurence of
  ”]      character "]".
    µ   Start new monadic chain. Value = "ab[cd","2ef".

Ḣ       Ḣead. "ab[cd"
 U      Upend. "dc[ba"
  œṡ”[  Split at first occurence of "[". | "dc","ba".

ẋ€        Repeat ...
  1¦        the element at index 1...
          by ...
    Ṫ Ḣ$    the Ḣead of the Ṫail of ...
          the input list ("ab[cd","2ef") (that is, 2)

          The command Ḣ also pop the head '2'. The remaining
            part of the tail is "ef".
     ©    Meanwhile, store the tail ("ef") to the register.

          Current value: "dcdc","ba"
FṚ        Flatten and Ṛeverse. | "abcdcd"
  ;®      Concatenate with the value of the register. "abcdcdef"

Çċ”]$¡    Main link.

 ċ”]$     Count number of "]" in the input.
     ¡    Repeatedly apply...
Ç           the last link...
            that many times.

Answer 18

C,381 bytes

Compact version:

while(1){int t=strlen(i);int a,c=-1;char*w;char*s;char*f;while(c++<t){if(i[c]==']'){int k=c-a;w=calloc((k--),1);memcpy(w,&i[a+1],k);s=calloc((t-c-1),1);memcpy(s,&i[c+2],t-c-2);i[a]=0;int r=i[c+1]-48;if(r==0){f=calloc(t,1);sprintf(f,"%s%s",i,s);}else{f=calloc((t+k),1);sprintf(f,"%s%s[%s]%d%s",i,w,w,r-1,s);}free(i);i=f;break;}else if(i[c]=='[')a=c;}free(w);free(s);if(c>=t)break;}

Full version:

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void proceed(char* input)
{
  while(1)
  {
    int t=strlen(input);
    int start,cursor=-1;
    char* word;
    char* suffix;
    char* final;
    while(cursor++<t)
    {
      if(input[cursor]==']')
      {
        int wordlength = cursor-start;
        word=calloc((wordlength--),sizeof(char));
        memcpy(word, &input[start+1], wordlength );
        suffix=calloc((t-cursor-1),sizeof(char));
        memcpy( suffix, &input[cursor+2], t-cursor-2 );
        input[start]='\0';
        int rep=input[cursor+1]-'0';
        if(rep==0)
        {
          final=calloc(t,sizeof(char));
          sprintf(final,"%s%s",input,suffix);
        }
        else
        {
          final=calloc((t+wordlength+5),sizeof(char));
          sprintf(final,"%s%s[%s]%d%s",input,word,word,rep-1,suffix);
        }
        free(input);
        input=final;
        break;
      }
      else if(input[cursor]=='[')
        start=cursor;
    }
    free(word);
    free(suffix);

    if(cursor>=t)break;
  }
}

int main()
{
  char* input=calloc(256,sizeof(char));
  sprintf(input,"a[[toto]2b]2[ana]3");
  printf("in : %s\n",input);
  proceed(input);
  printf("out: %s\n",input);
  return 0;
}

Answer 19

Python, 80 bytes

import re
b=re.sub
s=lambda x:eval(b(r"\](.)",r"')*\1+'",b(r"\[","'+('","%r"%x)))

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s("[Foo[Bar]3]2") Converts [Foo[Bar]3]2 to ''+('Foo'+('Bar')*3+'')*2+'', and evaluates.

Fails for input with quotes in brackets (e.g. [']3)