How do you access the matched groups in a JavaScript regular expression?

How do you access the matched groups in a JavaScript regular expression?

I want to match a portion of a string using a regular expression and then access that parenthesized substring:
var myString = “something format_abc”; // I want “abc”

var arr = /(?:^|\s)format_(.*?)(?:\s|$)/.exec(myString);

console.log(arr); // Prints: [” format_abc”, “abc”] .. so far so good.
console.log(arr[1]); // Prints: undefined (???)
console.log(arr[0]); // Prints: format_undefined (!!!)

What am I doing wrong?

I’ve discovered that there was nothing wrong with the regular expression code above: the actual string which I was testing against was this:
“date format_%A”

Reporting that “%A” is undefined seems a very strange behaviour, but it is not directly related to this question, so I’ve opened a new one, Why is a matched substring returning “undefined” in JavaScript?.

The issue was that console.log takes its parameters like a printf statement, and since the string I was logging (“%A”) had a special value, it was trying to find the value of the next parameter.


Solution 1:

You can access capturing groups like this:

var myString = "something format_abc";
var myRegexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
var match = myRegexp.exec(myString);
console.log(match[1]); // abc

And if there are multiple matches you can iterate over them:

var myString = "something format_abc";
var myRegexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
match = myRegexp.exec(myString);
while (match != null) {
  // matched text: match[0]
  // match start: match.index
  // capturing group n: match[n]
  match = myRegexp.exec(myString);

Solution 2:

Here’s a method you can use to get the n​th capturing group for each match:

function getMatches(string, regex, index) {
  index || (index = 1); // default to the first capturing group
  var matches = [];
  var match;
  while (match = regex.exec(string)) {
  return matches;

// Example :
var myString = 'something format_abc something format_def something format_ghi';
var myRegEx = /(?:^|\s)format_(.*?)(?:\s|$)/g;

// Get an array containing the first capturing group for every match
var matches = getMatches(myString, myRegEx, 1);

// Log results
document.write(matches.length + ' matches found: ' + JSON.stringify(matches))

Solution 3:

var myString = "something format_abc";
var arr = myString.match(/\bformat_(.*?)\b/);
console.log(arr[0] + " " + arr[1]);

The \b isn’t exactly the same thing. (It works on --format_foo/, but doesn’t work on format_a_b) But I wanted to show an alternative to your expression, which is fine. Of course, the match call is the important thing.

Solution 4:

In regards to the multi-match parentheses examples above, I was looking for an answer here after not getting what I wanted from:

var matches = mystring.match(/(?:neededToMatchButNotWantedInResult)(matchWanted)/igm);

After looking at the slightly convoluted function calls with while and .push() above, it dawned on me that the problem can be solved very elegantly with mystring.replace() instead (the replacing is NOT the point, and isn’t even done, the CLEAN, built-in recursive function call option for the second parameter is!):

var yourstring = 'something format_abc something format_def something format_ghi';

var matches = [];
yourstring.replace(/format_([^\s]+)/igm, function(m, p1){ matches.push(p1); } );

After this, I don’t think I’m ever going to use .match() for hardly anything ever again.

Solution 5:

Last but not least, I found one line of code that worked fine for me (JS ES6):

let reg = /#([\S]+)/igm; // Get hashtags.
let string = 'mi alegría es total! ✌?\n#fiestasdefindeaño #PadreHijo #buenosmomentos #france #paris';

let matches = (string.match(reg) || []).map(e => e.replace(reg, '$1'));

This will return:

['fiestasdefindeaño', 'PadreHijo', 'buenosmomentos', 'france', 'paris']

Solution 6:

Your syntax probably isn’t the best to keep. FF/Gecko defines RegExp as an extension of Function.
(FF2 went as far as typeof(/pattern/) == 'function')

It seems this is specific to FF — IE, Opera, and Chrome all throw exceptions for it.

Instead, use either method previously mentioned by others: RegExp#exec or String#match.
They offer the same results:

var regex = /(?:^|\s)format_(.*?)(?:\s|$)/;
var input = "something format_abc";

regex(input);        //=> [" format_abc", "abc"]
regex.exec(input);   //=> [" format_abc", "abc"]
input.match(regex);  //=> [" format_abc", "abc"]