As we???ve seen, a backslash \ is used to denote character classes, e.g. \d. So it???s a special character in regexps (just like in regular strings).

There are other special characters as well, that have special meaning in a regexp. They are used to do more powerful searches. Here???s a full list of them: [ \ ^ $ . | ? * + ( ).

Don???t try to remember the list ??? soon we???ll deal with each of them separately and you???ll know them by heart automatically.

Escaping

Let???s say we want to find literally a dot. Not ???any character???, but just a dot.

To use a special character as a regular one, prepend it with a backslash: \..

That???s also called ???escaping a character???.

For example:

alert( "Chapter 5.1".match(/\d\.\d/) ); // 5.1 (match!)
alert( "Chapter 511".match(/\d\.\d/) ); // null (looking for a real dot \.)

Parentheses are also special characters, so if we want them, we should use \(. The example below looks for a string "g()":

alert( "function g()".match(/g\(\)/) ); // "g()"

If we???re looking for a backslash \, it???s a special character in both regular strings and regexps, so we should double it.

alert( "1\\2".match(/\\/) ); // '\'

A slash

A slash symbol '/' is not a special character, but in JavaScript it is used to open and close the regexp: /...pattern.../, so we should escape it too.

Here???s what a search for a slash '/' looks like:

alert( "/".match(/\//) ); // '/'

On the other hand, if we???re not using /.../, but create a regexp using new RegExp, then we don???t need to escape it:

alert( "/".match(new RegExp("/")) ); // finds /

new RegExp

If we are creating a regular expression with new RegExp, then we don???t have to escape /, but need to do some other escaping.

For instance, consider this:

let regexp = new RegExp("\d\.\d");

alert( "Chapter 5.1".match(regexp) ); // null

The similar search in one of previous examples worked with /\d\.\d/, but new RegExp("\d\.\d") doesn???t work, why?

The reason is that backslashes are ???consumed??? by a string. As we may recall, regular strings have their own special characters, such as \n, and a backslash is used for escaping.

Here???s how ???\d.\d??? is perceived:

alert("\d\.\d"); // d.d

String quotes ???consume??? backslashes and interpret them on their own, for instance:

• \n ??? becomes a newline character,
• \u1234 ??? becomes the Unicode character with such code,
• ???And when there???s no special meaning: like \d or \z, then the backslash is simply removed.

So new RegExp gets a string without backslashes. That???s why the search doesn???t work!

To fix it, we need to double backslashes, because string quotes turn \\ into \:

let regStr = "\\d\\.\\d";
alert(regStr); // \d\.\d (correct now)

let regexp = new RegExp(regStr);

alert( "Chapter 5.1".match(regexp) ); // 5.1

Summary

• To search for special characters [ \ ^ $ . | ? * + ( ) literally, we need to prepend them with a backslash \ (???escape them???).
• We also need to escape / if we???re inside /.../ (but not inside new RegExp).
• When passing a string to new RegExp, we need to double backslashes \\, cause string quotes consume one of them.