javascript - Is it possible to modify flags on an existing RegExp? - Stack Overflow

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My code receives a RegExp object (out of my control). It isn't global but I need it to be.

At the moment I'm doing this:

if (!regex.global) {
  var flags = 'g';
  if (regex.ignoreCase) flags += 'i';
  if (regex.multiline ) flags += 'm';
  if (regex.sticky    ) flags += 'y';
  regex = new RegExp(regex.source, flags);
}

...because I can't figure out any other way.

• regex.global doesn't have a setter.
• regexpile(new_pattern) is deprecated in favour of new RegExp(new_pattern)
• regex.flags isn't a thing

Is there a better way to do this?

My code receives a RegExp object (out of my control). It isn't global but I need it to be.

At the moment I'm doing this:

if (!regex.global) {
  var flags = 'g';
  if (regex.ignoreCase) flags += 'i';
  if (regex.multiline ) flags += 'm';
  if (regex.sticky    ) flags += 'y';
  regex = new RegExp(regex.source, flags);
}

...because I can't figure out any other way.

• regex.global doesn't have a setter.
• regex.pile(new_pattern) is deprecated in favour of new RegExp(new_pattern)
• regex.flags isn't a thing

Is there a better way to do this?

• javascript
• regex

Share Improve this question Follow edited Jul 26, 2016 at 5:47 Rajeshwar 2,52744 gold badges3434 silver badges4949 bronze badges asked May 7, 2015 at 11:56 Julian TFJulian TF 45555 silver badges1010 bronze badges 4

• 4 Given that the internal structure of a regex depends, for example, on whether it's global or not (a global regex is an iterator), such a setter would be costly IMO (and what should it do ? Reset the state ?). Your solution is probably the best one. – Denys Séguret Commented May 7, 2015 at 12:01
• I'm going to risk it and ask - why do you need it to be global? – Benjamin Gruenbaum Commented May 7, 2015 at 12:12
• 1 @dystroy: you actually just saved me a future bug. Ie, if the regex IS global then I have to remember to reset regex.lastIndex to 0. So far as I can tell, state for a non-g regex and where sticky isn't implemented (mostly still the case) doesn't exist -> eg: var r = /a/; var s = "ababa"; r.exec(s); r.exec(s); // r.lastIndex always stays on 0 – Julian TF Commented May 7, 2015 at 12:23
• @BenjaminGruenbaum: it's part of a 'matchAll', returning all hits in the target string for that regex. I can do it without 'g' but that means maintaining the state myself (eg, repeatedly slicing the string after each match as there appears to be no way to tell the regex where to start from on each new non-global match). – Julian TF Commented May 7, 2015 at 12:25

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3 Answers 3

Sorted by: Reset to default 8

You can now do this in ES6. By "this" I don't mean modify the flags on an existing regexp, but rather create a new regexp from an existing one which replaces or modifies its flags. See http://www.2ality./2015/07/regexp-es6.html. Example from that post:

function copyWithIgnoreCase(regex) {
  return new RegExp(regex, regex.flags + 'i');
}

I'm afraid the answer is "No".

There is no way to change the flags on a existing regex. What you're doing seems to be the cleanest way.

You (arguably) could shorten the optional flags a bit:

var flags = 'g' +
    (regex.ignoreCase ? 'i' : '') +
    (regex.multiline ? 'm' : '') +
    (regex.sticky ? 'y' : '');

Even though you can't change the flags of an existing RegExp you can create a new one with modified flags. One nice option is using YourJS.flagRegExp():

rgx1 = /[A-Z]/g;

rgx2 = YourJS.flagRegExp(rgx1, 'i');
// > /[A-Z]/gi

rgx3 = YourJS.flagRegExp(rgx1, '!i!g');
// > /[A-Z]/i

The API documentation is found here. You will be able to customize your own JS library with that any other functions you want for your project.

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