How can I concatenate regex literals in Javascript?

Question

Is it possible to do something like this?

var pattern = /some regex segment/ + /* comment here */
    /another segment/;

Or do I have to use new RegExp() syntax and concatenate a string? I'd prefer to use the literal as the code is both more self-evident and concise.

Answer 1

You'll have to use new RegExp !-)

Answer 2

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 expression_one = /foo/;
var expression_two = /bar/;
var expression_three = new RegExp(expression_one.source + expression_two.source);

It's not entirely a good solution, as you lose the flags that were set on expression_one and expression_two, and is more wordy than just having expression one and two being literal strings instead of literal regular expressions.

Answer 3

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

Answer 4

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

Answer 5

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.