why is this javascript regex not match second and fourth pattern?

Question

the new_data output is ",$,2,$,4,$,"

but I am expecting ",$,$,$,$,$,", can someone kindly explain why to me? Thanks.

var data = ",1,2,3,4,5,";
var pattern = /,\d,/g
var new_data = data.replace(pattern, function(match){
  return ",$,";
});
console.log(new_data);

Avinash Raj · Accepted Answer · 2015-08-09 15:55:20Z

3

Because ,\d, eats up ,1, . Then the regex engine tries to match the next ,\d,, since the first pattern eats up the , which exists before 2 , ,2, wouldn't be visible to the regex engine. Likewise the same happens for 4.

So you could use positive lookahead.

string.replace(/,\d(?=,)/g, ",$")

Here the second comma won't be matched since lookarounds are assertions which won't capture any single character.

answered Aug 9, 2015 at 15:55

Avinash Raj

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2 Comments

aggressionexp Over a year ago

if I use ?: even if it doesn't capture the ",", it still eat up ",", seem only ?= lookahead not eating up any character, nice to know something new about regex, thanks @avinash-raj

Avinash Raj Over a year ago

yes (?:) (non-capturing group) should match the character .

CMPS · Accepted Answer · 2015-08-09 15:57:01Z

2

You can solve this by removing the last comma

/,\d/g

The problem is that the second comma is being consumed by the first match.

Example:

,1,2,3,4,5

Match 1: ,1,
,2, is not a match because the first comma is consumed by Match 1

Mathc 2: ,3,
,4, is not a match because the first comma is consumed by Match 2

answered Aug 9, 2015 at 15:57

CMPS

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2 Comments

dognose Over a year ago

but this would slihtly modify the meaning of the pattern. It would match, if it ends wiht ,5, not only if it ends with ,5,

CMPS Over a year ago

Yes you are right, but I am going to keep my answer because the OP might find this useful. Thank you for your comment @dognose

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