Escaping, special characters

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, such as [ ] { } ( ) \ ^ $ . | ? * +. They are used to do more powerful searches.
Don’t try to remember the list – soon we’ll deal with each of them, 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.

Original Content at: https://javascript.info/regexp-escaping

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