Here's a very useful regular expression cheatsheet which I've found, and it may be very useful to those who are not really familiar with regular expression, especially for PHP's preg_replace and preg_match functions.
| Regular Expression | Will match... |
|---|---|
| foo | The string "foo" |
| ^foo | "foo" at the start of a string |
| foo$ | "foo" at the end of a string |
| ^foo$ | "foo" when it is alone on a string |
| [abc] | a, b, or c |
| [a-z] | Any lowercase letter |
| [^A-Z] | Any character that is not a uppercase letter |
| (gif|jpg) | Matches either "gif" or "jpeg" |
| [a-z]+ | One or more lowercase letters |
| [0-9\.\-] | Аny number, dot, or minus sign |
| ^[a-zA-Z0-9_]{1,}$ | Any word of at least one letter, number or _ |
| ([wx])([yz]) | wy, wz, xy, or xz |
| [^A-Za-z0-9] | Any symbol (not a number or a letter) |
| ([A-Z]{3}|[0-9]{4}) | Matches three letters or four numbers |
Source: http://webcheatsheet.com/php/regular_expressions.php