Properly escape variable inside RegEx (JavaScript)

Question

Didn’t want to ask, but I gave up :-( I can’t find the solution

I can’t get how to escape the variable/string.

var highlight_chars_count = {{highlight_chars_count}};
var regex_string = "/(^\\w{" + highlight_chars_count + "})/";
var regex = new RegExp(regex_string);
$('h1').html(function(i, v) {
    return v.replace(regex, "<span>$1</span>");
});

This pattern works (without " and a variable)

var regex = new RegExp(/(^\w{2})/);

I think the solution is here JavaScript regex pattern concatenate with variable … but can’t transfer that to my regex.

The variable {{highlight_chars_count}} is a twig variable. Unfortunately I can’t insert the variable into the regex pattern either.

remove / in var regex_string = "/(^\\w{" + highlight_chars_count + "})/"; — Avinash Raj
– Avinash Raj, Commented May 10, 2016 at 14:10

Wiktor Stribiżew · Accepted Answer · 2016-05-10 14:10:44Z

4

You do not need / and you can do without the capturing group:

var regex_string = "^\\w{" + highlight_chars_count + "}";

and then

return v.replace(regex, "<span>$&</span>"); 
                                ^

Note that the regex delimiters (/.../) are necessary when you declare a regex with a regex literal notation when a regex is static (e.g. var rx = /abc/g). Here, you use a constructor notation.

Also, $& backreference refers to the whole match text, so, no need enclosing the whole pattern with a capturing group.

More information on RegExp regex literal and constructor notation at MDN

answered May 10, 2016 at 14:10

Wiktor Stribiżew

631k41 gold badges502 silver badges632 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

suntrop Over a year ago

Great tips! Thanks Wiktor.

Collectives™ on Stack Overflow

Properly escape variable inside RegEx (JavaScript)

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest