Question

Finding out whether a string contains abc could just as well be done with a call to indexOf . Regular expressions allow us to go beyond that and express more complicated patterns.

Say we want to match any number. In a regular expression, putting a set of characters between square brackets makes that part of the expression match any of the characters between the brackets.

Both of the following expressions match all strings that contain a digit:

console.log(/[0123456789]/.test("in 1992"));
// → true
console.log(/[0-9]/.test("in 1992"));
// → true

Within square brackets, a dash ( - ) between two characters can be used to indicate a range of characters, where the ordering is determined by the character’s Unicode number. Characters 0 to 9 sit right next to each other in this ordering (codes 48 to 57), so [0-9] covers all of them and matches any digit.

There are a number of common character groups that have their own built-in shortcuts. Digits are one of them: \d means the same thing as [0-9] .

\d	Any digit character
\w	An alphanumeric character (“word character”)
\s	Any whitespace character (space, tab, newline, and similar)
\D	A character that is not a digit
\W	A nonalphanumeric character
\S	A nonwhitespace character
.	Any character except for newline

So you could match a date and time format like 30-01-2003 15:20 with the following expression:

var dateTime = /\d\d-\d\d-\d\d\d\d \d\d:\d\d/;
console.log(dateTime.test("30-01-2003 15:20"));
// → true
console.log(dateTime.test("30-jan-2003 15:20"));
// → false

That looks completely awful, doesn’t it? It has way too many backslashes, producing background noise that makes it hard to spot the actual pattern expressed. We’ll see a slightly improved version of this expression later .

These backslash codes can also be used inside square brackets. For example, [\d.] means any digit or a period character. But note that the period itself, when used between square brackets, loses its special meaning. The same goes for other special characters, such as + .

To invert a set of characters—that is, to express that you want to match any character except the ones in the set—you can write a caret ( ^ ) character after the opening bracket.

var notBinary = /[^01]/;
console.log(notBinary.test("1100100010100110"));
// → false
console.log(notBinary.test("1100100010200110"));
// → true

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.