Question

A regular expression is a type of object. It can either be constructed with the RegExp constructor or written as a literal value by enclosing the pattern in forward slash ( / ) characters.

var re1 = new RegExp("abc");
var re2 = /abc/;

Both of these regular expression objects represent the same pattern: an a character followed by a b followed by a c.

When using the RegExp constructor, the pattern is written as a normal string, so the usual rules apply for backslashes.

The second notation, where the pattern appears between slash characters, treats backslashes somewhat differently. First, since a forward slash ends the pattern, we need to put a backslash before any forward slash that we want to be part of the pattern. In addition, backslashes that aren’t part of special character codes (like \n ) will be preserved, rather than ignored as they are in strings, and change the meaning of the pattern. Some characters, such as question marks and plus signs, have special meanings in regular expressions and must be preceded by a backslash if they are meant to represent the character itself.

var eighteenPlus = /eighteen\+/;

Knowing precisely what characters to backslash-escape when writing regular expressions requires you to know every character with a special meaning. For the time being, this may not be realistic, so when in doubt, just put a backslash before any character that is not a letter, number, or whitespace.

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.