Mastering JavaScript

Data Structures and Manipulation
Regular expressions
If you are not familiar with regular expressions, I request you to spend time
learning them. Learning and using regular expressions effectively is one of the most
rewarding skills that you will gain. During most of the code review sessions, the first
thing that I comment on is how a piece of code can be converted to a single line of
regular expression (or RegEx). If you study popular JavaScript libraries, you will be
surprised to see how ubiquitous RegEx are. Most seasoned engineers rely on RegEx
primarily because once you know how to use them, they are concise and easy to test.
However, learning RegEx will take a significant amount of effort and time. A regular
expression is a way to express a pattern to match strings of text. The expression itself
consists of terms and operators that allow us to define these patterns. We'll see what
these terms and operators consist of shortly.
In JavaScript, there are two ways to create a regular expression: via a regular
expression literal and constructing an instance of a RegExp object.
For example, if we wanted to create a RegEx that matches the string test exactly, we
could use the following RegEx literal:
var pattern = /test/;
RegEx literals are delimited using forward slashes. Alternatively, we could construct
a RegExp instance, passing the RegEx as a string:
var pattern = new RegExp("test");
Both of these formats result in the same RegEx being created in the variable pattern.
In addition to the expression itself, there are three flags that can be associated with a
RegEx:
• i: This makes the RegEx case-insensitive, so /test/i matches not only test,
but also Test, TEST, tEsT, and so on.
• g: This matches all the instances of the pattern as opposed to the default of
local, which matches the first occurrence only. More on this later.
• m: This allows matches across multiple lines that might be obtained from the
value of a textarea element.
These flags are appended to the end of the literal (for example, /test/ig) or passed
in a string as the second parameter to the RegExp constructor (new RegExp("test",
"ig")).
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