Inserting variables into a regular expression string

Question

I've found a lot questions on StackOverflow about how to do this, but it's just not working for me.

var username = "abcdefg";
var min_length = 3;
var max_length = 24;
var regex = new RegExp('^[\w.-]{' + min_length + ',' + max_length + '}$');
if (regex.test(username))
    document.getElementById('status').innerHTML = 'match';
else
    document.getElementById('status').innerHTML = 'no match';

I've also tried with slashes before and after:

new RegExp('/^[\w.-]{' + min_length + ',' + max_length + '}$/')

And I've also tried with double quotes instead of single quotes (not sure if regex treats single quotes as reserved or something).

If I remove the quotes and put in numbers for the variables it works fine:

new RegExp(/^[\w.-]{3,24}$/)

jsFiddle: http://jsfiddle.net/88ef3/

And you're sure that works if you just type the numbers instead ? — adeneo
– adeneo, Commented Jun 22, 2014 at 16:25

Oriol · Accepted Answer · 2014-06-22 16:27:15Z

3

When you use RegExp constructor to build a regular expression from a string, you must escape slashes:

new RegExp('^[\\w.-]{' + min_length + ',' + max_length + '}$');

Remember that a slash in a javascript string is used to escape, but here you want the slah as a character, so you must escape itself.

Demo

answered Jun 22, 2014 at 16:27

Oriol

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3 Comments

Gavin Over a year ago

That did it! Thank you. I'll accept this after the 10 minute period is up.

CrayonViolent Over a year ago

@Gavin also note from other comments that you do not use / in RegExp constructor, because the / is the delimiter for regex literals.

Gavin Over a year ago

That's what I figured, I just couldn't figure out why this wasn't working so I tried everything I could think of.

PeterKA · Accepted Answer · 2014-06-22 16:28:10Z

1

new RegExp('^[\\w.-]{' + min_length + ',' + max_length + '}$')

answered Jun 22, 2014 at 16:28

PeterKA

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1 Comment

Gavin Over a year ago

Thanks, but Oriol beat you to it :)

user3728375 · Accepted Answer · 2014-06-22 16:45:48Z

0

This may help

var username = "abcdefg";
var min_length = 3;
var max_length = 24;
var str1='^[\\w.-]{'+min_length+','+max_length+'}$';
var regex = new RegExp(str1,"i");
if (regex.test(username))
    document.getElementById('status').innerHTML = 'match';
else
    document.getElementById('status').innerHTML = 'no match';

http://jsfiddle.net/Et7J8/

answered Jun 22, 2014 at 16:45

user3728375

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