Why does the second part of this regex not match?

Question

var re = /\d(\d)/g;
var str = '123';
var match;
var results = [];

while (match = re.exec(str)) {
  results.push(+match[1]);
}
console.log(results);

Instead of [2, 3] as I would expect, it gives only [2] I cannot figure this out. Why doesn't the 23 match the regex and give the 3 as the capture group?

Are you thinking of overlapping matches? I don't think that works with regex — JCOC611, Jan 07 '19 at 23:13
because it starts again after the match, `var str = '1234';` would return 2, 4 — epascarello, Jan 07 '19 at 23:13
Ohhhhhhhh........is there a way to do overlapping matches? I suppose I would have to manually restart from the index following the start of the previous match, eh? — temporary_user_name, Jan 07 '19 at 23:13
https://stackoverflow.com/questions/20833295/how-can-i-match-overlapping-strings-with-regex — epascarello, Jan 07 '19 at 23:14
If you wanted the first part you could do it with lookaheads. Lookbehinds would work if JS supported them, but it doesn't. — Barmar, Jan 07 '19 at 23:15
Nothing like editing your question and removing half the content. lol — epascarello, Jan 07 '19 at 23:17
I removed baggage that I realized was unrelated to the crux of the question, neither contextualizing it nor deepening it, only distracting. What exactly are you complaining about? I did it as quick as I could, within 1-2 minutes I believe. — temporary_user_name, Jan 07 '19 at 23:20
@temporary_user_name I've updated my answer to be related to the updated version — mrzasa, Jan 07 '19 at 23:37
You can use `'123456'.match(/(?<=\d)\d/g)` on engines that support lookbehind assertions. — Patrick Roberts, Jan 07 '19 at 23:46

score 2 · Answer 1 · answered Jan 07 '19 at 23:58

This is because the string 12 is matched in the first run of exec, captured parenthesis is 2 which is appended to results variable and lastIndex property of re object is updated to 2

In the second run of exec, re.lastIndex is 2 which means begin matching source string at index 2 , So re has string 3 which clearly doesn't match the pattern /\d(\d)/ and exec returns null, while loop is exited and results array just contains 2

score 0 · Answer 2 · answered Jan 08 '19 at 01:36

0

How to do overlapping matches:

var re = /\d(?=(\d))/g;
var str = '123';
var match;
var results = [];

while (match = re.exec(str)) {
  results.push(+match[1]);
}
console.log(results);

answered Jan 08 '19 at 01:36

Why does the second part of this regex not match?

2 Answers2