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 insidenew RegExp
). - When passing a string to
new RegExp
, we need to double backslashes\\
, cause string quotes consume one of them.
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