Task with braces, brackets and parenthesis

Question

Task is to check if given string contains balanced sets of {}, [] and ().

For example, check(\"{[}]\") must return

Answer 1

This task is not doable with a regex because they have no memory and cannot remember the depth of parenthesing.

You should use a parser, probably a recursive descent parser, to do the task.

The grammar would look like this (not tested):

input: /* empty */
     | expr input

expr: paren-expr
    | bracket-expr
    | curly-bracket-expr

paren-expr: '(' input ')'

bracket-expr: '[' input ']'

curly-bracket-expr: '{' input '}'

Example code:

#include 
#include 
#include 


class parenthesis_checker {

  template 
  It parse_open_close(It first, It last, char op, char cl) const {
    if(op != *first) return first;

    It r = parse_input(first+1, last);
    if(last == r) return first;
    if(cl != *r)  return first;

    return r+1;
  }

  template 
  It parse_expr(It first, It last) const {
    It r = parse_open_close(first, last, '(', ')');
    if(r != first) return r;

    r = parse_open_close(first, last, '[', ']');
    if(r != first) return r;

    r = parse_open_close(first, last, '{', '}');
    if(r != first) return r;

    return first;
  }

  template 
  It parse_input(It first, It last) const {
    while(first != last) {
      It r = parse_expr(first, last);
      if(r == first) return r;
      first = r;
    }
    return first;
  }

public:
  template 
  bool operator()(It first, It last) const {
    return last==parse_input(first, last);
  }

  template 
  bool operator()(Cont value) const {
    return (*this)(value.begin(), value.end());
  }

  bool operator()(const char* str) const {
    return (*this)(std::string(str));
  }
};


int main() {
  parenthesis_checker check;

  std::cout << check("{[}]") << std::endl;
  std::cout << check("{[]()}") << std::endl;
}