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Complex Regular Expression Examples

User name check with regular expression

First start with a user name check. In case of a registration form you may want to control available user names a bit. Let’s suppose you don’t want to allow any special character in the name except “_.-” and of course letters and numbers. Besides this you may want to control the length of the user name to be between 4 and 20.

First we need to define the available characters. This can be realised with the following code:

[a-zA-Z0-9_.-]

After that we need to limit the number of characters with the following code:

{4,20}

At least we need to put it together:

^[a-zA-Z-0-9_.-]{4,20}$

In case of Perl compatible regular expression surround it with ‘/’. At the end the PHP code looks like this:

$pattern = '/^[a-zA-Z0-9_.-]{4,20}$/';
$username = "this.is.a-demo_-";

if (preg_match($pattern,$username)) echo "Match";
else echo "Not match";

Check hexadecimal color codes with regular expression

A hexadecimal color code looks like this: #5A332C or you can use a short form like #C5F. In both case it starts with a # and follows with exactly 3 or 6 numbers or letters from a-f.

So the first it starts as:

^#

the following character range is:

[a-fA-F0-9]

and the length can be 3 or 6. The complete pattern is the following:

^#(([a-fA-F0-9]{3}$)|([a-fA-F0-9]{6}$))

Here we use an or statement first check the #123 form and then the #123456 form. At the end the PHP code looks like this:

$pattern = '/^#(([a-fA-F0-9]{3}$)|([a-fA-F0-9]{6}$))/';
$color = "#1AA";

if (preg_match($pattern,$color)) echo "Match";
else echo "Not match";

Email check with regular expression

At least let’s see how we can check an email address with regular expressions. First take a careful look at the following example emails:

john.demo@demo.com
john@demo.us
john_123.demo_.name@demo.info

What we can see is that the @ is a mandatory element in an email. Besides this there must be some character before and some after it. More precisely there must be a valid domain name after the @.

So the first part must be a string with letters a numbers or some special characters like _-. In pattern we can write it as follows:

^[a-zA-Z0-9_.-]+

The domain name always have a let’s say name and tld. The tld is the .com, .us. .info and the name can be any string with valid characters. It means that the domain pattern looks like this:

[a-zA-Z0-9-]+\.[a-zA-Z.]{2,4}$

Now we only need to put together the 2 parts with the @ and get the complete pattern:

^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$

The PHP code looks like this:

$pattern = '/^[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,5}$/';
$email = "john123.demo_.name@demo.info";

if (preg_match($pattern,$email)) echo "Match";
else echo "Not match";
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Posted by on June 7, 2011 in PHP, Regular Expressions

 

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