And Slaters Go Plop

At least something
/.+/

A positive integer
/^\d*$/

A decimal number
/^\d*\.?\d/

A valid email address
/^\w+((-\w+)|(\.\w+))*\@[A-Za-z0-9]+((\.|-)[A-Za-z0-9]+)*\.[A-Za-z0-9]+$/

A valid web address
/^(http:\/\/)?\w+([\.-]?\w+)*\w+([\.-]?\w+)*(\.\w+)+(\/)?(\w+)?$/

A hexadecimal colour (6 chars)
/^#?[0-9A-Fa-f]{6}$/

How to use in Javascript:
var testString = "ff0000";
var pattern = /^#?[0-9A-Fa-f]{6}$/;
if (testString.match(pattern)) DoSomething();

How to use in PHP:
$testString = 'ff0000';
$pattern = '/^#?[0-9A-Fa-f]{6}$/';
if (preg_match($pattern, $testString) DoSomething();

How to use in C#:
string testString = "ff0000";
string pattern = @"^#?[0-9A-Fa-f]{6}$";
Match m = Regex.Match(testString, pattern);
if (m.Success) DoSomething();