Escaping, special characters

As we’ve seen, a backslash pattern:\ is used to denote character classes, e.g. pattern:\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 pattern:[ ] { } ( ) \ ^ $ . | ? * +. 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: pattern:.. That’s also called “escaping a character”. For example: Parentheses are also special characters, so if we want them, we should use pattern:(. The example below looks for a string “g()”: If we’re looking for a backslash \, it’s a special character in both regular strings and regexps, so we should double it.

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

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: The similar search in one of previous examples worked with pattern:/\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
, and a backslash is used for escaping. Here’s how “\d.\d” is perceived: String quotes “consume” backslashes and interpret them on their own, for instance: -
– becomes a newline character, - \u1234 – becomes the Unicode character with such code, - …And when there’s no special meaning: like pattern:\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 :

Summary

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

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