Form Validation with JavaScript Regular Expressions (Part 1)

Regular expressions are not just a feature of JavaScript; although I will be using JavaScript as an example, you will come across regular expressions in almost any programming language, including PHP, VB Script, ASP.NET, Java and PERL. Regular expressions are a powerful and versatile approach to form validation, so the knowledge gained here can be applied by any Web developer in almost any popular programming language.

The RegExp object is one of JavaScript's built in objects, and has methods and properties like the other built in objects: string, array and the like. You can create a new RegExp object in either a literal form or by using the RegExp() constructor.

The code for a literal regular expression is pretty basic; just define a variable whose value is encapsulated within single forward slashes:

var myRegexp = /www/;

This is a regular expression in its most basic form, and to go with it, we can make use of its most basic method; the .test method:

var myRegxp = /www/;

document.write(myRegxp.test("www"));

This would simply write 'true' to the page. There are a few other, very basic things we could do with this using flags. The regular expression in the above example is case sensitive, so changing the www in the first line to WWW would output 'false' instead. To make the regular expression case insensitive, you just add i directly after the initial declaration:

var myRegxp = /WWW/i;

document.write(myRegxp.test("www"));

which again, writes 'true'. Other flags that may be used are the g or global flag, and the m or multi-line flag.