Example of grouping concept and usage of javascript regular expressions

function matchDemo(){
  var s;
  //The expression is divided into three groups: d(b+)(d), (b+), (d) these three groups (actually four groups, including all the expressions themselves)  //The first bracket from the leftmost number is the first group, the second bracket is the second group, and so on, the corresponding values ​​are the values ​​of RegExp.$1 and RegExp.$2 respectively.  var re = new RegExp("(d(b+)(d))","ig");
  var str = "cdbBdbsbdbdz";
  //The value returned by exec() is an array that matches the expression search  var arr = (str);
  //The value returned by the expression d(b+)(d) is in line with the value returned by  s = "$1 contains: " + RegExp.$1 + ", RegExp.$1 : " + RegExp.$ + "\n";
  //The value returned by the expression (b+) is in line with the value returned by  s += "$2 contains: " + RegExp.$2 + ", RegExp.$2 : " + RegExp.$2 + "\n";
  //The value returned by this expression is in line with (d)  s += "$3 contains: " + RegExp.$3 + ", RegExp.$3 : " + RegExp.$3;
  //Get the position of the last character of the matching string in the string, that is, the starting position of the next match  alert();
  //If a value exists, the result of the last grouping is returned (returns the last submatch included in any regular expression search process)  alert();
  //Get the last matching string (returns the last matching character in any regular expression search process)  alert();
  //leftContext + lastMatch + rightContext == context
  alert();
  alert();
  //The result is the result of the second grouping  alert(RegExp.$2);
  return(s);
}
alert(matchDemo());

function matchDemo2(){
  var s,temp;
  //The result is divided into two groups (b+) and (d), and of course, it also includes all (d(b+)(d)) as the default groups.  var re = new RegExp("d(b+)(d)","ig");
  var str = "cdbBdbsbdbdz";
  //The result of arr includes the results returned by three groups (including all patterns).  //The two patterns of dbBd, bB, d and d match, while the second and third arrays actually match the two patterns (b+) and (d) based on the first one.  //In other words, the grouping is matched in a fully matched pattern, which plays the function of "filtering"  while((arr = (str)) != null)
  {
      alert(arr);
      temp = "$1 contains: " + RegExp.$1 + ", RegExp.$ : " + RegExp.$ + ",RegExp.$:" + RegExp.$; 
      alert(temp);
      //The $2 attribute means matching the second group, that is, (d) this pattern      s = "$2 contains: " + RegExp.$2 + ", RegExp.$ : " + RegExp.$;
      alert(s);
  }
}
matchDemo2();