As we’ve seen, a backslash
"\" is used to denote character classes. So it’s a special character.
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 it – when we deal with each of them separately, you’ll know it by heart automatically.
To use a special character as a regular one, prepend it with a backslash.
That’s also called “escaping a character”.
For instance, we need to find a dot
'.'. In a regular expression a dot means “any character except a newline”, so if we really mean “a dot”, let’s put a backslash before it:
alert( "Chapter 5.1".match(/\d\.\d/) ); // 5.1
Parentheses are also special characters, so if we want them, we should use
\(. The example below looks for a string
alert( "function g()".match(/g\(\)/) ); // "g()"
If we’re looking for a backslash
\, then we should double it:
alert( "1\\2".match(/\\/) ); // '\'
The slash symbol
/...pattern.../, so we should escape it too.
Here’s what a search for a slash
'/' looks like:
alert( "/".match(/\//) ); // '/'
From the other hand, the alternative
new RegExp syntaxes does not require escaping it:
alert( "/".match(new RegExp("/")) ); // '/'
If we are creating a regular expression with
new RegExp, then we need to do some more escaping.
For instance, consider this:
let reg = new RegExp("\d\.\d"); alert( "Chapter 5.1".match(reg) ); // null
It doesn’t work, but why?
The reason is string escaping rules. Look here:
alert("\d\.\d"); // d.d
Backslashes are used for escaping inside a string and string-specific special characters like
\n. The quotes “consume” 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
\z, then the backslash is simply removed.
So the call to
new RegExp gets a string without backslashes.
To fix it, we need to double backslashes, because quotes turn
let regStr = "\\d\\.\\d"; alert(regStr); // \d\.\d (correct now) let reg = new RegExp(regStr); alert( "Chapter 5.1".match(reg) ); // 5.1