Passing regex modifier options to RegExp object

I am trying to create something similar to this:

var regexp_loc = /e/i;

except I want the regexp to be dependent on a string, so I tried to use new RegExp but I couldn't get what i wanted.

Basically I want the e in the above regexp to be a string variable but I fail with the syntax.

I tried something like this:

var keyword = "something";

var test_regexp = new RegExp("/" + keyword + "/i");

Basically I want to search for a sub string in a larger string then replace the string with some other string, case insensitive.

regards, alexander



You need to pass the second parameter:

var r = new RegExp(keyword, "i");

You will also need to escape any special characters in the string to prevent regex injection attacks.


You should also remember to watch out for escape characters within a string...

For example if you wished to detect for a single number \d{1} and you did this...

var pattern = "\d{1}";
var re = new RegExp(pattern);

re.exec("1"); // fail! :(

that would fail as the initial \ is an escape character, you would need to "escape the escape", like so...

var pattern = "\\d{1}" // <-- spot the extra '\'
var re = new RegExp(pattern);

re.exec("1"); // success! :D

When using the RegExp constructor, you don't need the slashes like you do when using a regexp literal. So:

new RegExp(keyword, "i");

Note that you pass in the flags in the second parameter. See here for more info.


Want to share an example here:

I want to replace a string like: hi[var1][var2] to hi[newVar][var2]. and var1 are dynamic generated in the page.

so I had to use:

var regex = new RegExp("\\\\["+var1+"\\\\]",'ig');

This works pretty good to me. in case anyone need this like me. The reason I have to go with [] is var1 might be a very easy pattern itself, adding the [] would be much accurate.


var keyword = "something";

var test_regexp = new RegExp(something,"i");

You need to convert RegExp, you actually can create a simple function to do it for you:

function toReg(str) {
  if(!str || typeof str !== "string") {
  return new RegExp(str, "i");

and call it like:



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