Matching brackets in a string

Question

What is the most efficient or elegant method for matching brackets in a string such as:

\"f @ g[h[[i[[j[2], k[[1, m[[1, n[2]]]]]]]]]] // z\"

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Answer 1

Ok, here is another answer, a bit shorter:

Clear[replaceDoubleBrackets];
replaceDoubleBrackets[str_String, openSym_String, closeSym_String] := 
Module[{n = 0},
  Apply[StringJoin, 
   Characters[str] /. {"[" :> {"[", ++n}, 
     "]" :> {"]", n--}} //. {left___, {"[", m_}, {"[", mp1_}, 
      middle___, {"]", mp1_}, {"]", m_}, right___} /; 
       mp1 == m + 1 :> {left, openSym, middle, 
        closeSym, right} /. {br : "[" | "]", _Integer} :> br]]

Example:

In[100]:= replaceDoubleBrackets["f[g[h[[i[[j[2], k[[1, m[[1, n[2]]]]]]]]]]]", "(", ")"]

Out[100]= "f[g[h(i(j[2], k(1, m(1, n[2]))))]]"

EDIT

You can also use Mathematica built-in facilities, if you want to replace double brackets specifically with the symbols you indicated:

Clear[replaceDoubleBracketsAlt];
replaceDoubleBracketsAlt[str_String] :=
  StringJoin @@ Cases[ToBoxes@ToExpression[str, InputForm, HoldForm],
     _String, Infinity]

In[117]:= replaceDoubleBracketsAlt["f[g[h[[i[[j[2], k[[1, m[[1, n[2]]]]]]]]]]]"]

Out[117]= f[g[h[[i[[j[2],k[[1,m[[1,n[2]]]]]]]]]]]

The result would not show here properly, but it is a Unicode string with the symbols you requested.