转义元字符

Some characters in regex have a special meaning: they are called meta-characters. If you want to match the literal character (e.g. a real ., a real +, a real parenthesis) you need to prefix them with a backslash \, which escapes them.

The meta-characters to remember are twelve:

.  *  +  ?  ^  $  |  \  (  )  [  ]  {  }  /

(The / is not really an engine meta-character, but it must be escaped in the JavaScript literal notation /.../.)

Pattern: \$\d+\.\d\d
Sample:  Prezzi: $9.99, $12.50, totale $22.49.
                 ^^^^^  ^^^^^^         ^^^^^^

Here we escape $ (end-of-string anchor) and . (wildcard) to match the literal characters in the price format "dollar + number + dot + two digits".

Double backslash in JS strings

When you write a pattern inside a JavaScript string (like you do in our exercises) the backslash must be doubled. The JS string '\\d' contains two characters (\ and d) and is exactly the regex pattern \d.

const re1 = /\d+/; // literal notation: a single backslash
const re2 = new RegExp('\\d+'); // string: double backslash

The course editor shows the pattern already decoded (you see a single \d); you do not have to double anything yourself.

The 12 special meta-characters

The characters with special meanings in the engine are exactly 12: \\\ ^ $ . | ? * + ( ) [ {. If you want to match them as literal text, you must precede them with a backslash. In some environments (e.g. non-raw JS strings), you might need to double the backslash (\\\\).

Try it

Review exercise

Additional challenge