Use dynamic (variable) string as regex pattern in JavaScript

I found I had to double slash the \b to get it working. For example to remove "1x" words from a string using a variable, I needed to use:

    str = "1x";
    var regex = new RegExp("\\b"+str+"\\b","g"); // same as inv.replace(/\b1x\b/g, "")
    inv=inv.replace(regex, "");

If you are trying to use a variable value in the expression, you must use the RegExp "constructor".

var regex = "(?!(?:[^<]+>|[^>]+<\/a>))\b(" + value + ")\b";
new RegExp(regex, "is")

To create the regex from a string, you have to use JavaScript's RegExp object.

If you also want to match/replace more than one time, then you must add the g (global match) flag. Here's an example:

var stringToGoIntoTheRegex = "abc";
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;

var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.

JSFiddle demo here.

In the general case, escape the string before using as regex:

Not every string is a valid regex, though: there are some speciall characters, like ( or [. To work around this issue, simply escape the string before turning it into a regex. A utility function for that goes in the sample below:

function escapeRegExp(stringToGoIntoTheRegex) {
    return stringToGoIntoTheRegex.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
}

var stringToGoIntoTheRegex = escapeRegExp("abc"); // this is the only change from above
var regex = new RegExp("#" + stringToGoIntoTheRegex + "#", "g");
// at this point, the line above is the same as: var regex = /#abc#/g;

var input = "Hello this is #abc# some #abc# stuff.";
var output = input.replace(regex, "!!");
alert(output); // Hello this is !! some !! stuff.

JSFiddle demo here.

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Note: the regex in the question uses the s modifier, which didn't exist at the time of the question, but does exist -- a s (dotall) flag/modifier in JavaScript -- today.

You don't need the " to define a regular expression so just:

var regex = /(?!(?:[^<]+>|[^>]+<\/a>))\b(value)\b/is; // this is valid syntax

If value is a variable and you want a dynamic regular expression then you can't use this notation; use the alternative notation.

String.replace also accepts strings as input, so you can do "fox".replace("fox", "bear");

Alternative:

var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(value)\b/", "is");
var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(" + value + ")\b/", "is");
var regex = new RegExp("/(?!(?:[^<]+>|[^>]+<\/a>))\b(.*?)\b/", "is");

Keep in mind that if value contains regular expressions characters like (, [ and ? you will need to escape them.