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Escaping, special characters

As we've seen, a backslash "\" is used to denote character classes. So it's a special character in regexps (just like in a regular string).

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: pattern:[ \ ^ $ . | ? * + ( ).

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 a dot literally. Not "any character", but just a dot.

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

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 pattern:\(. 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:/...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("/")) ); // '/'

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 reg = new RegExp("\d\.\d");

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

The search worked with pattern:/\d\.\d/, but with new RegExp("\d\.\d") it doesn't work, why?

The reason is that backslashes are "consumed" by a string. Remember, regular strings have their own special characters like \n, and a backslash is used for escaping.

Please, take a look, what "\d.\d" really is:

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

The quotes "consume" backslashes and interpret them, 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 the call to new RegExp gets a string without backslashes. That's why the search doesn't work!

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

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

let reg = new RegExp(regStr);

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

Summary

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