Return positions of a regex match() in Javascript?

Question

Is there a way to retrieve the (starting) character positions inside a string of the results of a regex match() in Javascript?

Answer 1

Here's what I came up with:

// Finds starting and ending positions of quoted text
// in double or single quotes with escape char support like \" \'
var str = "this is a \"quoted\" string as you can 'read'";

var patt = /'((?:\\.|[^'])*)'|"((?:\\.|[^"])*)"/igm;

while (match = patt.exec(str)) {
  console.log(match.index + ' ' + patt.lastIndex);
}

Answer 2

exec returns an object with a index property:

var match = /bar/.exec("foobar");
if (match) {
    console.log("match found at " + match.index);
}

And for multiple matches:

var re = /bar/g,
    str = "foobarfoobar";
while ((match = re.exec(str)) != null) {
    console.log("match found at " + match.index);
}

Answer 3

From developer.mozilla.org docs on the String .match() method:

The returned Array has an extra input property, which contains the original string that was parsed. In addition, it has an index property, which represents the zero-based index of the match in the string.

When dealing with a non-global regex (i.e., no g flag on your regex), the value returned by .match() has an index property...all you have to do is access it.

var index = str.match(/regex/).index;

Here is an example showing it working as well:

var str = 'my string here';

var index = str.match(/here/).index;

alert(index); // <- 10

I have successfully tested this all the way back to IE5.

Answer 4

You can use the search method of the String object. This will only work for the first match, but will otherwise do what you describe. For example:

"How are you?".search(/are/);
// 4

Answer 5

Here is a cool feature I discovered recently, I tried this on the console and it seems to work:

var text = "border-bottom-left-radius";

var newText = text.replace(/-/g,function(match, index){
    return " " + index + " ";
});

Which returned: "border 6 bottom 13 left 18 radius"

So this seems to be what you are looking for.

Answer 6

This member fn returns an array of 0-based positions, if any, of the input word inside the String object

String.prototype.matching_positions = function( _word, _case_sensitive, _whole_words, _multiline )
{
   /*besides '_word' param, others are flags (0|1)*/
   var _match_pattern = "g"+(_case_sensitive?"i":"")+(_multiline?"m":"") ;
   var _bound = _whole_words ? "\\b" : "" ;
   var _re = new RegExp( _bound+_word+_bound, _match_pattern );
   var _pos = [], _chunk, _index = 0 ;

   while( true )
   {
      _chunk = _re.exec( this ) ;
      if ( _chunk == null ) break ;
      _pos.push( _chunk['index'] ) ;
      _re.lastIndex = _chunk['index']+1 ;
   }

   return _pos ;
}

Now try

var _sentence = "What do doers want ? What do doers need ?" ;
var _word = "do" ;
console.log( _sentence.matching_positions( _word, 1, 0, 0 ) );
console.log( _sentence.matching_positions( _word, 1, 1, 0 ) );

You can also input regular expressions:

var _second = "z^2+2z-1" ;
console.log( _second.matching_positions( "[0-9]\z+", 0, 0, 0 ) );

Here one gets the position index of linear term.

Answer 7

var str = "The rain in SPAIN stays mainly in the plain";

function searchIndex(str, searchValue, isCaseSensitive) {
  var modifiers = isCaseSensitive ? 'gi' : 'g';
  var regExpValue = new RegExp(searchValue, modifiers);
  var matches = [];
  var startIndex = 0;
  var arr = str.match(regExpValue);

  [].forEach.call(arr, function(element) {
    startIndex = str.indexOf(element, startIndex);
    matches.push(startIndex++);
  });

  return matches;
}

console.log(searchIndex(str, 'ain', true));