How to create a RegExp from its string representation in JavaScript?

Question

I have a string containing a regular expression in literal notation, e.g.:

var pattern = '/abc/i';

How can I create a regular expression object from that string?

Of course, I could just use eval:

eval(pattern); // /abc/i

But I'd like to avoid that.

Uh, new RegExp(pattern) ? -> developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — adeneo
– adeneo, Commented Apr 4, 2017 at 10:41
@WiktorStribiżew it's supposed to be a regex, but it could contain a typo. — Stefan
– Stefan, Commented Apr 4, 2017 at 10:43

anubhava · Accepted Answer · 2017-04-04 11:19:51Z

2

You can probably do something like this:

var pattern = '/ab*c/gi';

if ((m = /^\/(.*)\/([gim]*)$/i.exec(pattern)) != null) {
   restr = m[1], flag = m[2], re = new RegExp(restr, flag)
}
//=> /ab*c/gi

You may need to add handling of special regex meta characters if you want them to be treated literally.

edited Apr 4, 2017 at 11:19

answered Apr 4, 2017 at 11:02

anubhava

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1 Comment

Stefan Over a year ago

Escaping is not needed, the characters should be treated just like a regex literal. However, wouldn't it be easier to use capturing groups in order to match both parts at once, e.g. /^\/(.*)\/([gim]*)$/.exec(pattern)?

ibrahim mahrir · Accepted Answer · 2017-04-04 10:45:56Z

2

You can use a combo of substr and lastIndexOf like this:

var pattern = '/abc/i';

// the index of the first "/" is always 1
var index = pattern.lastIndexOf('/');          // index of the last "/"

var thePattern = pattern.substr(1, index - 1); // the actual pattern part
var theFlags   = pattern.substr(index + 1);    // the flags part

var regex = new RegExp(thePattern, theFlags);

console.log(regex);

answered Apr 4, 2017 at 10:45

ibrahim mahrir

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

I just created a function for that. Check it.

Substring from 1 ( to avoid first /) to last index of /). -> we got the pattenr.
substring from last index of / +1 to string length -> got the flags

Then pass the 2 arguments to the new RegExp function

function formRegEx(str){
  var reg=str.substring(1,str.lastIndexOf('/'));
  var flags=str.substring(str.lastIndexOf('/')+1,str.length);
  return new RegExp(reg,flags);
  
}
console.log(formRegEx('/abc/i'));
console.log(formRegEx('/[a-zA-Z]/ig'));

Update

From OP's comment on question, abc/i may present which should be treated as invalid.

So, a test can be done to check the validity

/^\/.*\/[igm]*$/

and then check for no existence of // by testing against

/[^\\]\/[^igm]/

The above function can be rewritten as

function formRegEx(str){
  if(!/^\/.*\/[igm]*$/.test(str)) return "Invalid RE";
  if(/[^\\]\/[^igm]/.test(str)) return "Invalid RE";
  var reg=str.substring(1,str.lastIndexOf('/'));
  var flags=str.substring(str.lastIndexOf('/')+1,str.length);
  return new RegExp(reg,flags);
  
}
console.log(formRegEx('/abc/ig')); // true
console.log(formRegEx('/abc//i'));  // false
console.log(formRegEx('/a/bc/i')); //false
console.log(formRegEx('/[a-zA-Z]/ig')); // true
console.log(formRegEx('abc/ig'));  // invalid
console.log(formRegEx('abci')); // invalid
console.log(formRegEx('/abc/ik')); // invalid

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