Question

The following code writes “response” as expected:

var str = "Would you like to have responses";
var pattern = "Response";
document.write(str.match(new RegExp(pattern,'gi')));

but when I add a specifier like \b, the following writes “null”:

var str = "Would you like to have responses";
var pattern = "\bResponse";
document.write(str.match(new RegExp(pattern,'gi')));

How can I make it work?

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Interestingly, both of the following works

document.write(str.match(new RegExp(/Response/gi)));
document.write(str.match(new RegExp(/\bResponse/gi)));

but I want to use new RegExp(pattern,modifiers); syntax.

Answer 1

A backslash has a special meaning in a string, it’s the escape character. When you want to create a Regular expression from a string, backslashes has to be escaped.

In the case when the escaped characters have the same meaning, you’re not going to notice it:

 '\n'.match('\n');
 '\n'.match('\\n');

both work. However, \b does not have such a meaning in a string.
new RegExp("\bResponse"); becomes /Response/ (\b is a backspace character, which does not represent a visible character). When an escape does not have a special meaning, the backslash is dropped.