How can I concatenate regex literals in JavaScript?

Here is how to create a regular expression without using the regular expression literal syntax. This lets you do arbitary string manipulation before it becomes a regular expression object:

var segment_part = "some bit of the regexp";
var pattern = new RegExp("some regex segment" + /*comment here */
              segment_part + /* that was defined just now */
              "another segment");

If you have two regular expression literals, you can in fact concatenate them using this technique:

var regex1 = /foo/g;
var regex2 = /bar/y;
var flags = (regex1.flags + regex2.flags).split("").sort().join("").replace(/(.)(?=.*\1)/g, "");
var regex3 = new RegExp(expression_one.source + expression_two.source, flags);
// regex3 is now /foobar/gy

It's just more wordy than just having expression one and two being literal strings instead of literal regular expressions.

I don't quite agree with the "eval" option.

var xxx = /abcd/;
var yyy = /efgh/;
var zzz = new RegExp(eval(xxx)+eval(yyy));

will give "//abcd//efgh//" which is not the intended result.

Using source like

var zzz = new RegExp(xxx.source+yyy.source);

will give "/abcdefgh/" and that is correct.

Logicaly there is no need to EVALUATE, you know your EXPRESSION. You just need its SOURCE or how it is written not necessarely its value. As for the flags, you just need to use the optional argument of RegExp.

In my situation, I do run in the issue of ^ and $ being used in several expression I am trying to concatenate together! Those expressions are grammar filters used accross the program. Now I wan't to use some of them together to handle the case of PREPOSITIONS. I may have to "slice" the sources to remove the starting and ending ^( and/or )$ :) Cheers, Alex.

You can concat regex source from both the literal and RegExp class:

var xxx = new RegExp(/abcd/);
var zzz = new RegExp(xxx.source + /efgh/.source);

No, the literal way is not supported. You'll have to use RegExp.

the easier way to me would be concatenate the sources, ex.:

a = /\d+/
b = /\w+/
c = new RegExp(a.source + b.source)

the c value will result in:

/\d+\w+/

I prefer to use eval('your expression') because it does not add the /on each end/ that ='new RegExp' does.