What does the d flag mean in a javascript RegEx?

Question

I have a regex, that appears to work in Safari and Chrome that gives the following error in FireFox.

Error: invalid regular expression flag d
Source File: http://java.net/projects/mq
Line: 194, Column: 34
Source Code:
    var vbkpatt1=/projects\/[^/]+/di;

I had fought with this RegEx a couple weeks ago and had put it aside, so I do not have a link to the page that led me to use the 'd' flag. A pointer to a ref that includes the d flag would be a solid start to resolving my issue.

Webkit will happily ingest /string/zxcvbgi, ignoring the invalid zxcvb, and still applying the valid gi flags. Not so Firefox. — Ken Redler
– Ken Redler, Commented Dec 11, 2010 at 4:51
"If the match succeeds, the exec() method returns an array (with extra properties index, input, and if the d flag is set, indices; see below...) " is a ref. However, if you rely on 'd' to get indices and need them, your code won't work without out, making the selected answer unsuitable. It's 2021 and the 'd' flag is documented by not actually present on most platforms. — Craig Hicks
– Craig Hicks, Commented Aug 10, 2021 at 21:40
Judging from your code, you don't need the indices, because you aren't using subgroups. — Craig Hicks
– Craig Hicks, Commented Aug 10, 2021 at 21:42

Andrejs · Accepted Answer · 2022-10-04 12:29:57Z

7

Since ECMAScript 2022, there IS a "d" flag.

From MDN Web Docs: The d flag indicates that the result of a regular expression match should contain the start and end indices of the substrings of each capture group. It does not change the regex's interpretation or matching behavior in any way, but only provides additional information in the matching result.

const regexWithOutDFlag = /hi(\d)/g;
const hi = 'hi1hi2';
const result = [...hi.matchAll(regexWithOutDFlag)];
console.log(result[0]);

// output:
[ 'hi1', '1', index: 0, input: 'hi1hi2', groups: undefined ]

const regexWithDflag = /hi(\d)/gd;
const hi2 = 'hi1hi2';
const result2 = [...hi2.matchAll(regexWithDflag)];
console.log(result2[0]);

// output:
[
  'hi1',
  '1',
  index: 0,
  input: 'hi1hi2',
  groups: undefined,
  indices: [ [ 0, 3 ], [ 2, 3 ], groups: undefined ]
]

answered Oct 4, 2022 at 12:29

Andrejs

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2 Comments

lukstafi Over a year ago

It doesn't seem to work in TypeScript though... (at least not for me)

kshetline May 23 at 10:56

Thank you for saying flat-out, "It does not change the regex's interpretation or matching behavior in any way". The W3 documentation doesn't not make that clear, and the "Try it" example provided doesn't show the indices (which are the whole point) in any way.

Ken Redler · Accepted Answer · 2010-12-11 16:10:23Z

2

Webkit browsers are more tolerant in this case, and will accept something like this:

/theregex/zxcvbgi

Instead of throwing am error, they see it as:

/theregex/gi

Firefox, however, will object to any invalid flags. Nick points out the valid ones in his answer.

answered Dec 11, 2010 at 16:10

Ken Redler

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Comments

Community · Accepted Answer · 2022-02-03 23:16:43Z

1

There is no d flag, which is your issue :) There are:

g - Global search (multiple matches)
i - Case insensitive
m - Multiple input

edited Feb 3, 2022 at 23:16

CommunityBot

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answered Dec 11, 2010 at 4:41

Nick Craver

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3 Comments

Craig Hicks Over a year ago

developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… --- "If the match succeeds, the exec() method returns an array (with extra properties index, input, and if the d flag is set, indices; see below) ...". At the bottom of the page it shows all browser (even IE) and nodejs supporting. HOWEVER - using the 'd' flag in nodejs throws an error. Without the 'd' flag you can get the text (and the length, which is redundant info) of an arbitrary group in the match, but not the starting index of the group.

Craig Hicks Over a year ago

This answer has information about a proposal "currently at stage 3", meaning it will eventual be part of JS, that will provide the indices as well. My guess is that that the 'd' flag is that, included in the documentation prematurely, but maybe my guess is wrong.

Mr Lister Over a year ago

According to MDN, it was implemented in Firefox v88 and Chrome v90.

Matthew Flaschen · Accepted Answer · 2010-12-11 04:52:13Z

0

Are you sure it actually does something in Chrome? I tried:

/projects\/[^/]+/zyx

It accepts that as /projects\/[^/]+/, but I strongly doubt those are all real extensions. It's just ignoring them. as noted by Ken, it will keep valid flags even if invalid ones are present.

Also, I recommend you follow a good tutorial, rather than just cutting and pasting.

edited Dec 11, 2010 at 4:52

answered Dec 11, 2010 at 4:43

Matthew Flaschen

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