var st1 = "8,12,17,20"; 
var st2 = "8,12";
  
console.log(st1 ); //expected output (8-12, 17-20)
console.log(st2);  // expected output (8-12);

I want to replace all alternative commas (of index position 1,3,5,7...) with hyphens using regex as shown in the example above. Thanks in advance.

CodePudding user response：

We can try the following approach:

var inputs = ["8,12,17,20", "8,12", "8,12,17"];
var outputs = inputs.map(x => x.replace(/(\d ),(\d )/g, "$1-$2"));
console.log(outputs);

The logic is simply to find each pair of numbers, from the start, and replace the comma with a hyphen. By definition, this will spare the even numbered commas in between number pairs.

A demo for earlier versions of JavaScript:

var inputs = ["8,12,17,20", "8,12", "8,12,17"];
for (var i=0; i < inputs.length;   i) {
    var output = inputs[i].replace(/(\d ),(\d )/g, "$1-$2");
    console.log(inputs[i]   " => "   output);
}

CodePudding user response：

You can replace matches of the following regular expression with a hyphen:

,(?=(?:(?:\d ,){2})*\d $)

This matches commas that are followed later in the string by an even number of commas.

Demo

This expression can be broken down as follows.

,           # match comma
(?=         # begin a positive lookahead
  (?:       # begin a non-capture group
    (?:     # begin a non-capture group
      \d ,  # match one or more digits followed by a comma
    ){2}    # end non-capture group and execute it twice
  )*        # end non-capture group and execute it zero or more times
  \d        # match one or more digits
  $         # match end of string
)           # end positive lookahead

CodePudding user response：

    var st1 = "8,12,17,20,3,4,5,6"; 
    var st2 = "8,12";
    
    //string to array
    var array1=st1.split("");
    
    var listIndex=[];
    var z=0;

   //Find all the comma from the array 
   function FindComnmaIndexlist(array1){
        for(let i=0;i<=array1.length;i  ){
    
            if(array1[i]==','){
                    listIndex[z  ]=i;
                    array1[i]='-';
            }
        
        }
    }
    // replace function
    String.prototype.replaceAt = function(index, replacement) {
        if (index >= this.length) {
            return this.valueOf();
        }
     
        return this.substring(0, index)   replacement   this.substring(index   1);
    }
    
    // call find command index
    FindComnmaIndexlist(array1);
    
    
     // replace the comma
    var nwts=st1;
    for(let i=0;i<listIndex.length;i  ){
        let v=listIndex[i];
        if(i%2==0){
         nwts=nwts.replaceAt(v,'-');
        }
        
    }
    
    //print 
    console.log(nwts);